I had planned on using .080" thick material for my build. Here is the link to the US Plastic's page:
http://www.usplastic.com/catalog/product.asp?catalog%5Fname=usplastic&category%5Fname=115&product%5Fid=10432
You can probably get away with .120", but you will have to account for the extra material thickness, you can make the parts a little longer and trim the excess once you have the pieces assembled.
Hope this helps.
Alan
Kripps - That is looking goood!! I may just have to give the batting helmet a try myself.