We used heavy slotted shelving material to make the frame. This also came from Grainger - but it was left over from other projects.
We brazed a large hex nut onto a hub that mounts on the shaft and we turn it with a large socket on the end of a speed wrench.
One change I would make - we mounted the bearings to a couple of pieces of slotted angle and it is too flexible - hard to align the bearings so they don't bind. Plus, the bolts holding the slotted metal are also the ones that have to be loosened and tightened to adjust the belt. I would replace that with a solid block of metal and then mount that block to the frame.
It took 2 of us about 4 hours to make.