How can I safely sand a small, thin item with a belt/disc sander?
Make a piece of board that looks like the following with some small 0.1 to 0.2 inch thick strips glued to it. This board would be long enough that you can hold it safely near the belt sander. The pocket in the middle will be the periphery size of the small item that you want to sand.



Stick a piece of double stick tape into the cavity bottom to hold your piece in place. Now you can hold the part up against the belt sander with ease. The length of the board will also help to eye the angles and keep it straight to the sander. 

It may even be possible to use the double stick tape alone on the board without the containment cavity.