1 old ball mouse
1 peanut butter lid (yes, really)
a bunch of small coins
some old Legos:
-1 four or six-peg axle
-2 small right-angle gears
-1 large wheel
-some spacer bits
Step 1: Cheap Spinner Interface
Just cut out the bare parts you need. That means keep the plastic that holds the encoder shaft in place, and try to leave enough extra to support the electronic bits. It doesn't hurt to have a good flat area to glue the whole thing down with either.
Step 2: Attach the Shaft Gear
Stick the gear on the shaft, leaving enough room for the mating gear. The tricky part is to keep the gear centered on the shaft. If you spin it while the glue is setting, you can tell what parts are off and gently correct them.