Mattress Underlayment

And as a possibly cheap option, there's that black plastic breather/ weeping sheet stuff that's put against house foundations, before back filling, that lets the concrete breath and moisture to escape. Kinda looks like small egg crate with cloth glued to the bumpy side. Might be able to find some off cuts. A couple pieces would be easier to wrestle then a one piece sheet too.
I used this in an old basement floor, under plywood for air circulation. Worked a treat. Was free from a few house build sites in the neighbourhood.