I happen to use this tool all the time in the shop, whether it is just knocking burrs off a cut piece of wood, or what it is really good for...trapping a bow. I made another one today and took some pics for a short build along.
This is easy to make, and you will use it alot....just make sure you take your time to get it sized right and tight.
First, take 3 x 24 sandpaper belt (i like to use an old one as they are slightly stretched) and a block of wood a little longer than you need. I am using a scrap piece of actionboo here.
Cut it to size to fit the width of the belt (i.e. 3"). Then take it to the router and round off one end.
Then put one end of the paper just slighly on the block and mark out where you think the cut needs to me. Cut it, but cut it a little long.
Then back to the router and route the other side...if it is a little too long, take it to the belt sander to just take a bit off. Then back to the router to round off the edges.
When it is so close that the paper will almost go on, use your random orbital sander (or any sander to take a little bit off one just one side (so the paper be pushed on easier on that side)