Being an aircraft mechanic, I end up working in all kinds of body positions, standing, sitting, lying down, upside-down, and reaching overhead.
To save my knees, sometimes I use a mechanics rolling seat, instead of kneeling.
Sometimes I work on a bench, but my rolling seat is too short to sit and work from at a bench.
The shop only has one, and it is normally in use, plus it is bare metal and uncomfortable.
I could'vebought one, but that's just against my nature.

Recently one of my coworkers had a mechanics seat that the welds failed on, so he gave it to me for scrap metal.
At the same time, an office chair broke, and it was by the door to be thrown away.

Step 1: Dr Frankenstein

I removed the comfortable top from the broken mechanics seat, saving the hardware.
Then I removed the seat bottom from the broken office chair, the fasteners required an Allen wrench, but I have a tool box full of tools.
The office chair base was still solid, and the wheels are perfect.
I lined up two of the holes in the office chair base with two screw holes in the mechanics seat top. The holes were too large, but a washer under each screw head solved that problem.
The pictures of the black chair are from my second version. My neighbor was throwing out anow old worn out office chair, however the wheels and height and tilt parts worked just fine.
I also had an old mechanics creeper seat that the wheel bushings wore out on. A replacement set of bushings was more than the chair cost when new.
I unbolted the old office chair seat, and the old mechanics creeper seat base.
These seats are also used on lawn tractors, you might find one that way as well.
The rubber cover snaps off the seat pan, and the foam is usuallyonly held by a few spots of RTV or adhesive.
I had to drill new holes through the seat pan and seat post bracket, and added a few 1/4" bolts I had lying around.
Then glue the seat foam in place.

Step 2: It's Alive!

Now I have a perfect work stool for bench work, and any overhead projects under the aircraft.
The second version, with the larger seat will remain at work, it is height adjustable, and reclines, the adjustment lever will also lock the seat from reclining if I want it to.
Notice the picture of the almost identical black creeper seat next to mine. That seat cost over $125 US, and it doesn't recline.
I'm still working on a parts tray design, I'll add that as soon as I make it happen
The first version, with the red top, will go to my sawfish kayak construction studio, it will be exactly what I have been looking for.
Total cost, free!