Second option would be stall mats. For horses. These are hard rubber mats we put on the stall floor. Bay's standing on one in the picture.
They're made out of tire-grade rubber, like kongs are, or harder, and as much as 3/4 inch thick. They're heavy, the dogs wouldn't be able to pull them up or chew them... you could cut them to fit with a saws-all or a circular saw, but it's not easy. One of them costs about $40. They'd have them at your local farm supply in a few different grades and thicknesses.
They'd be a little more comfy to lay on than the hard plastic.