The quick fix for desk height is 4 or 8 concrete blocks. You can get half-thickness concrete blocks to make the height 0.5 or 1.5 blocks. For most people, 2 blocks lying flat with the holes horizontal is enough for standing at the desk.
At Amazon, for their legendary door desks, the solution was almost this. You'd open a facilities ticket and eventually a guy with a glue gun and slices of 4x4s in various heights would show up. He'd pick the slice height necessary to get your desk to the right height, place a drop of glue on each slice, crawl under your desk and raise it with his shoulders while sliding the slices into place under the legs.
