I'd used one before myself. I used a type of clear/amber electrical tape called furon that is made to pass heat through without passing electricity. I covered the bottom of the heatsink with the tape, then used white thermal paste to set it in place (I don't lan, the box doesn't move). I use type 44 thermal paste on all my stuff, and only keep AA adhesive.
It got warm enough still to need a fan on it, so it must've been working.
If you don't have furon tape (most don't, it's $40 per roll) you could give your heatsink a double coat of polyeurethane spray to coat it electrically. As long as your particular video card doesn't have any sharp protrusions to scrape through the coating. Coatings don't have much effect on thermal transfer except when dealing with a mighty cpu heat.
Anodizing works as an insulator also, but don't use it if it's scratched, as that spot will conduct.
Heck, scotch tape might work if it'll survive the temperatures that the heatsink will reach.
And again, DO NOT try this if your card can scratch the coating, your card will die! This is at your own risk, as I cannot control how good of a job you do.
Frag tape is a good option too, as it shouldn't pass electricity either.
