Poly works. I'd let it air out first. To me, it has that "plastic" smell right out of the bag. The downside to polyfill is when it gets wet, it will freeze up, and basically be like a rock under the pan. If it works, don't change it.

I personally cut my own from screen wire. I cut it to the shape of my trap jaws, with tabs that let me tuck it under the jaws. This also effectively increases the "kill area" of the pan. It also has the affect of reducing the pan tension a tad. I also use waxed paper that I'll roll up tight and spread back out to cover the trap. In certain instances this works better than the screen wire. I make that call at the set.