I welded up the relief cuts I made in the corners, then I rounded off the corners of the frame and boxed them.
I then ground down the pinch welds and rewelded the frame with butt welds. As you can see I left a small bit of the pinch weld at the bottom of the frame so I would have something to clamp to for final adjustments, this will be ground down once I have welded the frame in place.
Here is a good shot of the new rounded frame.

I boxed in the corners but left the center un-boxed. The interior side and back of the frame will be wrapped in vinyl and I will make a matching removable filler piece for the center of the frame. When I make the top I will design it so its front lip fits into the center slot (with the filler piece removed) for a secure and fairly water tight fit.
I boxed in the corners but left the center un-boxed. The interior side and back of the frame will be wrapped in vinyl and I will make a matching removable filler piece for the center of the frame. When I make the top I will design it so its front lip fits into the center slot (with the filler piece removed) for a secure and fairly water tight fit.