Been awhile and wish I could remember the exact dimensions but a race car fabricator friend made his up with wheel cylinders and a 2000 lb pressure guage. Had a 1 ft square curved plate the tire rested on and would press down on the cylinder, Just sit the car on plates and read the guage. He said he had maybe $30 in all four.
He was a big dirt track guy from Indiana and used them from the 50s till he died.