I use Avery 5430 plain white labels 1 3/4" x 3/4" (about 500 for $6)--
sufficient room for my illegible scrawls. If you were super-paranoid
about UV, though, you could pick up some silver foil mailing labels
(Avery 8986) 2 1/4" x 3/4" and snip them in half. But they're $13
for 300 labels, which isn't much of a deal.
I'm sure your local office stationer's will have more suggestions...
For those DIY-ers, spray some Scotch 77 adhesive on the barrier of
your choice and stick it on...
--Chuck