"Large airplane chocks" are what came to mind, and whaddya know, cvairwerks had drawings!
For a tractor, I'd build something similar out of the largest hunks of wood I could find, stacked and lagged together if you're using lumber, or chainsawed out of logs. Since access to the back of the wheel is more difficult with a tractor, the rope tying them together in the back would have to be left out.
If you'll be using them on soft surfaces much, maybe screw cleats to the bottom?
Also, if things are getting ridiculously heavy, maybe a wheel at one end, placed off the ground such that when you pick up the other end you can roll it around like a wheelbarrow.
Back when I helped out on the farm, we usually used assorted logs. A log with a handy stub of a large branch sticking out so it wouldn't roll was especially prized.
We also had large hunks of lumber lying around for cribbing and chocking. It's a good use for stuff too rough to use for construction.