I tried the 1 inch belt sander, but found it difficult to control,  and
couldn't do the sharp notches with it. I used a fine polishing grit of
sandpaper (320), but it burned the plastic and cut way too fast. If I tried
to sand the sharp notch, the edge of the sanding belt cut into the long side
of the plastic.  If someone has been able to use this successfully, I'd sure
like to know how you do it.

The router with a lamination trimming bit seems the way to go.