Golflover, I've heard people say that you should level to all sorts of places:
1) Use the floor of the freezer
2) Get it level to where you're comfortable (possibly even with slides out) and then mount the level according to that
3) Use the bottom of the frame to level against
4) Picture of water on a counter or simply a level on a counter
All seem like good possibilities. There was a thread yesterday about figuring out where level is. I know it can be a challenge, I can change how level we are or aren't simply by moving the level on the floor.