Although my work bench is very flat I was not happy with the how unlevel it sat from end to end due to the slope in my garage floor - about a 2" drop from front to rear. I created some levelers using extra materials from the shop: 2x4, 3/8" x 3" carriage bolts, hex nuts and washers. Simply drilled a 1" dia pocket 3/8" deep in the end of four 2x4's (8" long), drilled a 3/8" dia hole 3" deep, soldered nuts to washers (I know, there are flange nuts but this was an excuse to fab something from metal parts!) and screwed in the bolts. I ground (actually burned) round pockets in two pieces of 2x4 and two pieces of 3/8" plywood to serve as feet. Now I can easily level the bench single-handedly using a 3/8" wrench. With just a few turns on the bolts my bench is now level side-to-side and end-to-end with no twist. The work surface previously came up to the top of my thighs, now it is closer to hip-high which should be fine.
