I've always cut scrap 2x12 into different lengths... Then screwed them together, longest on bottom to shortest on top.
Say for 3 high, I use 30", 24", and 20" for the front footprint, and double side by side for the back footprint... Then I use my handy dandy dial level from harbor freight. Sometimes I don't need to rest on the top board, but somewhere in between. Air brakes help with that!
But, I also have an obsession with gathering scrap wood wherever I can.
And if soil is very impressionable, I cross another 2x12 at 90° to make a platform
I once complained I had no shoes....
Until I met a man with no feet