I used two layers of Therm O Web (Heat n' Bond) Gloss
It's actually meant for fabric, but worked just fine on paper. The directions say to iron it, but DON'T
. It's low-tack and very easy to apply -- if you make a mistake, just move it. I used a flat shell (button front) that I don't currently stock, but I will if people like it!
Time for a tutorial...