Actually the BOC from Joanns works fine on a 54" frame. There is plenty of stretch to staple at least a 1/2" beyond your frame.
I'd be sure to use quite a bit of support when building this. I'd maybe even use some luan plywood or some thin gauge aluminum in the corners and butt joints to insure the structual integrity of the thing.
I built mine with 2X4's around the outside and 2x3's in the inside as support - this way the internal supports do not show behind the stretched fabric.