Yep, closed cell foam sleeping mats are perfect to craft your own pads.
They come in several thicknesses. Don't take up mositure. And are (relatively) cheap.

I have cut up a few of those too to create custom inserts for various bags and packs. Use pads to put between things stacked in too-deep compartments in regular photo bags too.
Well worth considering if you want to add some padding to a unpadded bag.