I made a floor for my 4-person Ice Cube out of a 4' x 8' sheet of 7/16" OSB (because it's fairly inexpensive and quite easy to work with).
I cut a 2 ft piece off one end (2' x 4') then made a 12" square cut-out along one long edge of that to allow for a fishing hole. It looks something like this:
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I then cut the remaining 4' x 6' piece in half up the middle to make two 2' x 6' pieces. One piece I left alone, and the other I made a couple more 12" square cut-outs to allow for more fishing holes. There's enough room between the holes for a camping chair, and 13" on each end that accommodates the coke/milk crate that my 20lb propane tank sits in. I then attached those two boards back together with a couple of hinges so that they could be folded up into one reasonably small unit for easy storage in my 6 1/2 ft truck box.
With that folded out plus the smaller board along side it all looks something like the next figure, of course depending on if or how I want to align the the smaller board:
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It's not too heavy, although I don't normally use it when I'm going to walk or sled out on the ice ... usually just leave it packed-up in the pickup truck box then pull out anywhere that I can drive out to for ice fishing. 3/8" OSB would be only slightly lighter weight.
Again, that's from one 4' x 8' sheet. I came across another 2' x 6' section of the same 7/16" OSB, so constructed a second identical outer-side board with the hole cut-outs. Now it completely covers the Ice Cube floor, of course except for the hole cut-outs, and looks like this:
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