The 'Apoxiesculpt' buildup will likely work fine, but I have always preferred to extend areas like this with fiberglass, as it is much stronger. A couple ways would be to either add a few layers overlapping from inside the neck, down. Then do the fine shaping with thickened gelcoat or bondo. Or do the entire neck-extension in fiberglass... nice and thick and solid as a rock. Then a little cleanup and bodo where needed. ...Just another suggestion...

No need to take it down. I provided that as an educational reference for members of The Prop Den.

Vendors who inappropriately use my photos, however, will eat lightsaber.

Would this be about the correct position to build it out to?
I will build it up against the card and when it's dry build the back up. Then I will be a bit forcefull with it and pull it about to make sure it stays there.

The side corners of the neck bother me too... just seem to poke out pretty far...

It all determines on the skill and effort that is wanted to be put into this. If it is just a quick fix to get it somewhat back into original DP CA shapes, then the attached white outline is pretty much what should be aimed for. If it is to make the neck accurate... it would likely require a complete re-sculpt of the whole neck to make them straighter and more accurate. I tried drawing up an accurate neck and overlaying it onto the picture of this helmet... and the work needed is substantial.

Was made from the best angle I could find matching this picture. Not perfect.

Ok update for you.
I have had a bit of a serious problem with altering the neck on this.
I had planned to heat up the sides and slowly bring them in, then extend the neck line. However when I was bending the neckline I had a MASSIVE crack starting to form in the gelcoat going all the way through. So after a major repair job and lots of additional material in the neck region I have decided to leave the neckline as it is.
I am a little bit gutted but even with uber mods it still wouldn't of been really accurate.
So here I am now getting it ready for paint.
It's still an exciting project for me though as it's my first helmet and I am learning a great deal of things. I may look out for a anh beater and have a go at totally transforming it.
I will post pics when I start the painting. I may document the entire painting process and post in the guide section so others will be able to follow it and not make the same mistakes I will undoubtedly make.

In all honesty, I think people obsess over the "neck flare" and try to manipulate it to make the neck more narrow, only to do it incorrectly in most to all cases. In the end, I think the aesthetics aren't necessarily improved to where the effort was worth it.

With a shorter neck, you have a reduced surface area if combining this with armor. Reducing the neck flare will reduce this surface area even further, making it incompatible with most available armor.

A lot of people also miss out on the fact that at certain angles (looking downwards) the perception of the neck flare will naturally increase. This angle will vary depending on the degree of actual flare and the length of the neck.

I would encourage you to simply achieve something that is aesthetically balanced. Some people try to force things to be more "accurate" without taking into account whether the entirety will be improved, or will the modification look unnatural based on the mask's shape.

I'd say: just restore it to a Don Post's shape. That's actually what I'm doing with a really hideous project. Focus on adding the material to the front lip of the neck that was cut down by the person who made your mask.

Ok now I have done a test fit on my manequin I can see why the neck flaire is there and the neck has been trimmed.
For some reason the helmet allmost catches on the armour. If not for the issues with it then it would probably sit on the armour. Hmmmmmm
What can be causing this? If you look the eyes (not actually pointing the right way) of my mannequin they are level with the eyes in the mask. Is this correct?
Could it be that the mask is slightly oversized or (what I suspect) that there is not enough internal padding?

You'll probably need to pad your manequins head to get a snug fit with the helmet so it sits nicely.

Yeah Allthough I remembered to put the padding in the top I completely forgot about all of the other internal padding I ripped out!! whoops!!!!

Please try standing back a minimum of 6 feet when photographing, and zoom in without cropping the costume parts. Too much lens distortion going on here, but I think I see the issue.

Your mask is not oversized. The mannequin head is simply too small, and its neck is probably really thin and longer than what most human beings have - if the mannequin is the typical kind that came off eBay.

You may consider wrapping some bubblewrap to pad the neck, and then some black material to obscure the bubble wrap. If your mask's fiberglass is brittle and/or thin, don't go too overboard with neck reinforcement, else it will require flexing the mask to get it on the mannequin, and material strain may cause your mask to crack in weak areas (depending on how well it was made).

Well, Dave Prowse has a long neck, so the long necks on the mannequins are not so much an issue, imo...

I'm talking proportions. Prowse's neck was probably wider than the mannequin's head.

Ok after much fu*#ing about with the neck I decided to break off the resin around the neck as I just couldn't get to look right and leave it as it was.
I have decided that when I do a major project on a lid it will be on one that will benefit most from the time I put into it. As this will never be movie accurate I will leave it as it is.
So here it is fully painted up with some metal tusks. I am quite pleased with how it turned out and it sits well on my mannequin.
You will see I have added some Velcro to the front as I cut down the mount it made it a little loose so this keeps it in place.

HOLY CRAP, that turned out nice. Really looks like the face mask and dome was painted together. Looks awesome. Very well done.