@vaughn - Yep, decent tubes are in short supply these days. Used to be able to pick 'em up off the road all over the place. I've still got one or two around. Rubber roofing is similar but really not thick enough unless they're making thicker than .060 somewhere. Two to three times that thickness is just right. Light and flexible enough to fit anywhere and still thin enough to transmit a bit of solar energy to the air space below. I've also been thinking cheapie rubber floor mats but they can be very stiff. Some are extremely stiff.
As far as "insulating" your hole air space: I'm in the other camp whereby the use of an uninsulated thinner material can actually accumulate solar energy and heat the airspace between the cover and the water. Yep, more sunlight is better and less, well, not so much.
I cut 14" squares; sometimes I'm in 10" holes. They flop in the bottom of the sled or slip into the bucket on the side, conforming to the bucket shape occupying basically zero space.