Wouldn't the plate just get covered with stuck leaves etc. giving the snails
a bridge to cross? Maybe this isn't a problem where you saw them used
before?

Apparently coffee works well. 2% concentration kills 92% of slugs and
snails. 0.1% is enough for a deterent ( a double strength cup of coffee ). I
haven't tried myself but my father-in-law regularly puts used coffee grounds
out and says they seem to be working.

Martin