|Building this was relatively easy, but here are a few points:
Using a 2" hole saw in my Milwaukee drill, I cut two holes in the top end of the square steel tube, about 3" from the top edge. This hole is to allow the back side of the flange mount bearings to sit into the tube and let the flange sit flush on the steel. Since these type of bearing are self aligning, the two holes don't have to be alginged perfectly, but I was able to get them within 1/64" of each other.
Using a 1/2" drill bit I drilled two holes in one side about 5" from the bottom edge. Using a metal cutoff wheel on my 4-1/2" grinder, I cut three lines. Two vertically from the bottom edge of the tube up to each of the holes and another horizonally between the two holes. This produced a square cutout in the tube along the bottom which allowed me to easily mount the motor with the shaft and pulley inside the tube.
The rest was pretty much simply drilling a few screws here and there to mount the tube on the bottm pieces of angle, and attached the castors and motor.
The only trick to all this is securing the drum to the shaft. The price of a machined shaft that had threaded ends, either internal or external to the shaft, was $80 compared to a plain shaft at $20. So I went with the plain shaft and shaft collars. The way to get shaft collars to grip the drum tightly so the drum doesn't slip when the shaft turns wasn't hard once I figured it out. What I did was mark the spot on the shaft where the set screw of the collar would touch the shaft. Then using a file and my Dremel, I filed out two notches, tapering inwards toward where the drum would be. So when I slide on the drum, I place the collars are tight as I could to the drum. But then when I tighted down the collar set screw, the screw hit the tapered cutout and shifted the collar towards the drum as I tightened. This held the drum on the shaft as tightly as if the shaft had a thread and I used a large bolt.
(diagram to come)