I just go down to the local sign shop and have them cut my AMA number out of the thin plastic numbers. I usually place the AMA number on the outboard wing up near the leading edge (as far out as looks decent) to keep it out of the fuel exhaust stream. Nice professional looking job....just a little expensive.

I use 2" Tahoma bold font. I have never lost a number yet due to fuel. By the way, they will also cut any nice graphics for you as long as you email it to them or bring it in on a disk.