I bought a $20 4ft x 8ft sheet of pink Owens-Corning R-5. It's about an inch thick.
I use it over screened inner lids, inside telescoping covers.
My screened bottom boards are raised on attached pressure-treted cleats to allow a West Beetle tray.
So I also cut foam board sized to slide between the cleats instead of the trap, great to keep nucs wam in early spring. Or to protect big hives from chill during tropical storms.
And I cut some to go inside a feeding hive body rim, when transitioning a nuc to a 10-frame. I used a 4" hole saw to drill the center of the foa board. This goes on top the unscreened inner lid. When the quart jar feeder is on, the bees drink syrup instead of chewing the foam. They seem to prosper with the extra thermoregulation.
And a larger piece of foam is useful for putting under a hive body when pulling brood. When new bees fall off, the foam catches them and it's a breeze to lift. One light lift of the board and light tap...and fallen nurse bees slide into the hive at close-up.