Here's some things I've designed and made using various steel tubing. I only have an acetyline outfit, so some of these have taken a while to put together. These are all prototypes and they are being tested in my world.
1. Backhoe Dolly -
Made with 2x2 steel and 2.5" casters. Very simple and it works great.
2. Brush Forks. Made with 1x2 and 2x2. Very stout and useful. Have these on most of the time. Mount with one bolt on each side so it pivots up and you can use the bucket if necessary. Use for brush and also they can do cultivation, move rocks, etc.
I welded the forks to the back piece, then put the 2x2 spacers in between.
3. Mechanical Thumb
Always wanted a thumb, but don't use it much because the forks do about everything I need. Based on the Bro-Tek thumb; I had a local fabricator make the side plates from 5/16 steel from a template I gave them (cost $150). The thumb is 1x2 (3) and I used the stock 3 pt. hitch top link. I used grade 5 parts and 3/4" bolts instead of 1" which I think is overkill based on the forces that the backhoe can produce. Works great and I have about $200 into it. Would go for grade 8 bolts if I was going to use it a lot.
4, Shovel Holder
No welding here. Just 2x2 steel. Uses existng bolts that mount the turn signal. Bolted a piece of 3/4" angle inside the bottom as a stop. Works great - may add another one on the other side.
Forks and Thumb at work: