I have a motorcycle, and a helmet that I might want to upgrade, but Im not sure what you mean by conversion. Ive never seen that done.
So far thinking about doing the buzzaxe to compliment the psycho, or another mask, since they are small and whatnot. Ive never been big into half-life, but maybe something from another game. I need to go through my game library and think about what badass ideas are in there. Its always tempting to do halo, but having done one MC helm, I can definitely say its more than I can tackle right now.
My other thought was to do the other bandit mask, this one. The costume I have matches it more than the psycho mask
PS, just found this article, wow.....
http://barnyardfx.blogspot.com/2012/09/borderlands-2.html