The Heatsheet that I was trying to fold (several times) got plenty of abuse without tearing or puncturing.

(Still haven't tried the vacuum thingy. I need a new, not yet unfolded 'sheet. Know I got one around here somewhere.)

ziplock bag + hard chair + you = Ham pressed gear, the low cost vacuum pack.

They are pretty small as is. I've never seen someone fold something like this smaller than the factory did.

IR, we're just trying to get it back down to something close to factory folded size. Once you unfold it the extra air is more difficult to get out than you think. It gets trapped in the folds and such. Worse with one of the bivvys. I hesitate to use the ziplock solution you suggested as I wonder if the compressed air with nowhere to go could possibly compromise the bag, an improvised whoopee cushion if you will.

Actually, I'm trying to get mine thinner than factory. The ones I have are on the "puffy" side. Flattening it on a table with something heavy over it might work, over time. Too much weight too quickly I would fear causing a blowout. I have more hope for the vacuum thingy.

(hopefully I'll find the other un-unpacked 'sheet sometime over the weekend to give it a try)

I've used the vacuum thingy on stuff like a pack-towel and spare winter socks. Pretty sure I got them more compact than the "Ham Press" ever would have.