This is a neat thread. Inspired me to take my new and developing welding skills and build a welding cart with them.
Completely overbuilt and weighs 100+ lbs but it was a fun project and it works! Used 1 1/2 by 2 1/2" rectangular tubing (0.120) with a 12ga sheet for the bottom. Also learned how to strip zinc coating off steel with muriatic acid to make the wheels safe to weld on the bottom. Muratic acid is some nasty stuff.
Basically made an identical top and bottom. Then cut the corners so the vertical supports would overlap a bit. Probably more work than necessary but everything lined up. I ground flat most of the exterior welds so it had a nice finished look to it, but kept some on the inside seems. Maybe if I get bored I will give it a paint job in the future.