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Try armaflex pipe insulation. You can get it at Homans in Williston. It comes in A 4'x4' sheet. Just cut your own circles. 1 inch thickness works the best!
I've used them since they banned alcohol in the holes. My first ones were black rubber,which worked almost too good. I've seen sunny days where they would melt an inch or more down into the ice. The ones I use now are made out of double foil bubble insulation They work great. I can go a whole day without skimming a hole, and they don't weigh much more than a pound for the whole set.
BigWiggle I think I got 14 out of that mat. For 6-8 bucks it was a good deal. I took my pattern and marked the out and cut them out with scissors. The mat was a little over 20" x 60" I think. I like your tip about packing a little slush around the edge, the snow will find any little gap and get in when it is blowing.