blocks for leveling under jacks and tires

I use yellow legos under my jacks. If on quite slanted site, I might put 3 under the left side jacks and one under the right side jacks...for example. I've never had a problem with breakage. For tires that may come off the ground, I carry two large pieces of wood that are long enough and wide enough for the left and right rear dual tires to be fully on the wood. Of course, I can use the wood for the front tires too.