Similar but I recently customised my motorcycle crash helmet. I went to a local graphics shop (look in yellow pages) and they custom made me some vinyl stickers. I could have anything I wanted in any size and colour and they made them and sent them out the next day.
The adhesive was safe to use on a helmet too and if you wanted to remove them they don't leave a sticky mess.
Also, I am not familiar with the GPA and it's outer shell but if it's safe to do so you could get someone to airbrush a design on it. Must find out if it's safe to do that though and wouldn't damage the shell.
Good Luck and if you get something done come back with pics!