Peter,
I Googled benchtop drum sander to get an idea of what an industry standard machine looks like and found this term covers several variations or configurations of sanding machines.
Are you looking to make one that is used in a similar manner as a thickness planer?, or one with an exposed belt and a small shelf or table to permit sanding at 90 degrees to the face of a component?
This has me interested in finding a solution, 
Tom


Edit: I just used YouTube to search for "How to make a drum sander", the results were plentiful. Give this a try, good luck.