For a sensitivity test like that, the back of the leg at the bend or the inner arm will probably work best. It has to suck to be allergic to both latex and plastic. I'm allergic to latex and that's bad enough, you add plastic to that and its mind blowing because everything that is made latex-free for people with allergies is usually plastic. I don't know how the hell you cope. I know you can get silk based condoms from Europe but from what I've read they are even less reliable than the sponge.