the only thing i can think of is to make a liner out of oilcloth.
perhaps if you framed the inside(make it more squared off) in cardboard or foam core? then lined that with something waterproof?

this is so amazing!
 
you did a fabulous job!
<3

are you using the freezer paper method? because two colors can get complicated. but i do like the idea of making the black the outlines. looks wicked cool. i might steal that idea.
if youre thinking of doing green outlines on the shirt. more simple, the best way to  remember is that everything thats black will then be green, and everything thats white would be black
kinda like this:
http://www.craftster.org/pictures/data/500/medium/234233_15Nov10_dark_mark_ii.png