This sketch is drawn to scale, but if printed follow the measurements given. Do not rely on a scale.
The base can be made any dimension that the builder wants to suit his/her needs. I mounted the motor on the base as a previous design in which another maker mounted his motor on the side of the box proved to be unstable initially. He had to enlarge the base.
The top can be made to any size that the maker wants. The basic box consists of 2-3/4"x10"x10" end pieces and 2-3/4"x19-1/2"x10" (diagram needs to be corrected) birch plywood side pieces. This allows for a 4"dia.x17-9/16" sanding drum. How did I come up with such an odd number for the length of the drum??? When I put the drum together for a dry run I found that 24 MDF "donuts" measured approximately 18-1/4" which wouldn't work with a 24" long shaft unless I went with 1/4" material on the ends of the box, which I did not want to do. I also wanted to mount the bearings on the outside of the box. The original design had the bearings mounted on the inside of the box, but I prefer to see them mounted outside of the box and its sawdust. When I removed one MDF donut I had a measurement of 17-1/2" which after gluing and clamping ended up at 17-9/16". This also allowed me more "working space" on the sheave side of the shaft where I wanted it.
I just happened to use Browning 3/4" bore VPLS-212 pillow block bearings as I found a good price on a pair of them. I also liked the better finish of their cast iron. You can use any bearings that you find suitable.
As the holes in the ends of the box are drilled slightly oversize for the 3/4" bearing shaft, I installed felt washers on the inside of the box to contain the dust better, even though very little dust probably would come out without the felt washers.
I buy my stainless steel bearing rod from Speedy Metals online. By the time I drive to a local metal supplier and drive back I find it easier to use Speedy Metals and about the same cost when everything is factored in. Even so, the bearing shaft is not inexpensive at almost $50 delivered.
I bought the link belt at Harbor Freight with a 20% off discount coupon. That made a 5 foot length cost about $21 :-)
By mounting the sheaves as shown this allows one to conveniently use a straight edge to get them in the same plane.