To make sure the project I am building is square, on top of the work bench on one corner perfectly square, I glued and screwed two pieces of wood 5/8"X2"x 2'. When I make a hive body, I glue, set it inside the corner of the strips , clamp and screw. It comes out square and no twists every time. To keep from getting glue on the table top or into the corner, put a piece of plastic like schrink wrap or Saran wrap over the strips and onto the table top.
When you want to use your belt sander to sand, you can use these strips as stops to hold the piece of wood you are sanding.