Sorry, I can't help you out with personal experience cause I don't run footholds, but the guys I know that do a lot prefer wax paper or metal screen. Probably get more suggestions over on tman.
i've never used underalls but have used polyfill..
i year i used it for the whole season. found it to be kind of a pain in the ass.
usually ended up with some sticking out and above the pan. had to find a stick to poke the stuff down under the pan.
also had problems with mice digging it up.