I used nutserts, also called rivet nuts by other manufacturers
Rivet Nut Set. Much like a rivet but you're setting a nut into a bigger hole, crimp it in place, then use a stainless steel button head screw to attach the cup washer to the trailer. I think I used #10's. You need to be careful as the sidewalls are pretty darn thin and the Filon is fairly fragile, take your time drilling and tighten the screws by hand. I had to drill out the washers for the bigger screw, maybe you can get away with a #8. I've done roughly 8 or them over the years.