Yesterday I cut some 3/4" B-C grade plywood to size for the top plate, then laminated it to a piece of scrap motley brown Formica I had used on my work benches. The slot will not be cut until I decide on the drum diameter.
Lowes only had 1-1/2" galvanized (or the black) pipe and the rigid or EMT electrical pipes were just 2" diameter. I was looking for 2-1/2" or 3" galvanized pipe. It seems to me that a heavier wall steel pipe will have an advantage over thinner wall pipe due to flywheel effect.
I'll see if a local welding shop has something of use. The plan is to use some 3/8" HRS plate I have on hand to machine three donuts to fit inside the pipe, at the middle and at each end. They will be MIG welded in place.