Ok, I've built one of these jigs -- henceforth I'm going to call it "The Grasshopper" in honor of its inventor -- and I think I've ruined my flat.
How did those of you who built one keep the flat parallel when pounding it into the frame? Mine tilted to the side.
If I can borrow a camera I'll post pictures.
I tried hammering on the flat itself to straighten it out and now I have gouges... sigh