Jerry,
There are four 3/16th holes in the top of each bed rail and I think that's how it was originally attached. I adapted your suggestion and bolted a 1x3 cedar board to the top of each rail through those existing holes then I'm
going to attach aluminum angle to that. I can then bolt the frame to that. I used camper sealer tape between the rail and cedar. I'm thinking I can attach snap hardware to the cedar and to the canvas top.
That's the plan for now anyhow.
