I made this tool rest for the 4" belt sander. First a prototype with plywood and plastic, then the official release from 1/4" mild steel. Both versions have a 90 degree arm bent from 1/4" aluminum. The second shot shows both arms together for comparison.
The intended application is grinding plane blades. Using a bevel gauge, set the tool rest to an exact angle to the belt and clamp the adjustable handle. Next pinch a plane blade on the tool rest with your opposable thumbs and apply light pressure up into the revolving belt. Try to keep it straight into the belt. The beauty of this method is there is no need to measure or mark any guide lines. In seconds you will have a rounded, cambered bevel roughly at the angle you set, ready for honing.