Small Step Stool With Folding Legs Butler Hinges?

[Alan Smithee]

I need to make a small stool out of 3/4 plywood stock with folding legs. The top will be about 12"X15" and the legs about 6" tall. I want the legs to fold inward and flat when not in use but swing out at 90 degrees, AND LOCK, when the stool is set up. Is there such a hinge? I tried looking at some "bulter tray" hinges but they all start at 180 degrees and then go to 90. I need a hinge that goes from 0 degrees to 90. The other thing I considered was using somehow pinning the legs when they are at 90 (open) but I don't know what I'd use. Help appreciated.

[bridger]

the lock is a flap that folds down from the center of the step and is assisted by gravity.