I bought a "barn stall mat" from a local farm and ranch store. It's 4 x 6 feet, 3/4 of an inch thick, and made of old, ground up tires.
The intent is to cut it up into a set of 12 x 24" and 18 x 24" rectangles for use under the front and rear wheels, respectively. The word "intent" means that I've had the mat for sime time, but haven't got around to the cutting it up.
I saw a "how-to" over on IRV2 and it looked like a good idea. It's cheap at less than $50 for the mat. Provides enough rectangles for plenty of lift when needed, and is pretty much indestructible. The only downside is that it is pretty heavy.
They should work great, once I get them cut up...
Regards,
Randy