I have used aluminum drywall stilts for several costumes, with as much as a 24" lift. The trick is to get used to wearing them first, and you should do fine. I had 18" stilts last year, bar hopped all night with people bumping into me, even played pool with no problems (I'm 6'3"). I've also built my own stilts, but there is really no comparison. You want those suckers as light as humanly possible, especially if you're wandering around a con all day, and poorly home-built ones can seriously hurt you.
If you can't borrow some, I would recommend renting a pair of aluminum drywall stilts. Some are at a set height, while others can be adjusted quite a bit. I can't think of the brand mine are, but any commercial grade stilts should work great. They rent pairs around here for $25/day (as compared to $300 for mine new)
Good luck with it!