I saw a product at either Lowes or Home Repot last weekend made just for that.

They look like 2'x2' OSB panels (oriented strand board) that have 1/2" diameter x 3/8" tall (maybe 1/2"), round, black, rubber, bumpers all over the bottom of them. The edges of them have tongues and grooves on them. You just set them on the concrete, tap them together with a rubber mallet, and then carpet over the whole works.

I cannot for the life of me remember the name of the product.

M- M60s/VP150/QS8s/SVS PC-Ultra/HK630 Sit down. Shut up. Listen.