

Have you tried DAS clay? If you’re British, you can buy it pretty cheap from The Works, Hobbycraft or The Range.
It seems like it’d be exactly the right consistency to fill out these molds - a smidge firmer than playdoh, but much softer than plasticine.
Edit: just remembered, you can even get stone effect DAS clay for the same price, so no painting needed!
If you don’t mind a one-use option, lots of wrapping paper has 1 inch squares printed on the back. Just buy a cheap roll and use the other side.
It rolls up and the marks don’t accidentally rub off. Once you’re done with it, just pop it in the recycling.