I have an 8 year old Son who loves Marvel and loves cosplaying. I'm not very good / experienced at this sort of thing, but I bought the toy mask (it is just a mask, it comes with a little blaster as well, and has holes where the eyes are, which looks terrible).
I used 11x17 craft foam, glued 3 or 4 sheets together, then put the mask on my Son's face and wrapped the foam around his head and marked it, then glued it to both sides of the mask. I then drew the design of the helmet on the foam and cut it down to size/shape. I slapped several layers of Elmer's glue / Sticky Glue / water on it to stiffen it up. Then primed and painted it.
For the eyes, I bought a $12 pair of sunglasses that had reddish lenses in them, popped the lenses out, drimmeled them down a bit and then affixed them to the inside of the mask.
Considering I've never done anything like this before, I was really happy with the results. My Wife also made the trench coat, based on a children's sewing pattern for a bath robe. She worked off of that to get the basic design and then altered it to our liking. Then made a shoulder belt out of leather-ish fabric, painted up a couple of those orange toy blasters, and wa-lah!
Hope this helps. I'm not on RPF all that often so feel free to PM me if you have any questions. Good luck! My Son looooooves this costume.
Oh, and we went to the GotG red carpet premiere in Hollywood, and James Gunn took one look at the costume, and immediately came over to talk to us for a couple of minutes. It was awesome!
I already have the cheap-o plastic mask, and blasters--got them for Halloween this year, so was looking for something a bit more realistic. Given this though, I think I can totally make that work. Didn't even think of using that as a base, and adding foam around it. Off to the craft store I go!
I have been building my son's Star Lord costume with the Disney Store long jacket as the base foundation. The best child size full helmet mask we bought are from X-Coser as a DIY kit. Buy it without the Glow Glass lights. It does need some work like some cutting, reshaping with a hairdryer as it ships crushed a tad. Once this is done primer and paint for the screen accurate look. I have progression pics on my site. Awaiting a compact battery box to add the micro string red LED lights and the red sunglass lens with screen door mesh for the final look.