I made a pattern of the curve at the rear of top opening. I fit wood pieces at the rear where top meets rear panel. Adjusted pattern to be 1/4 inch lower.I then made the cross struts from 2 pieces of flat steel, the bottom one remaining flat and the upper one curved to the pattern. Clamped around the pattern and to a straight, strong steel piece I welded the top and bottom piece with a few uprights pieces of the same flat steel. Struts are bolted to side roof panel lip. My plan is to tap the steel struts to #10 for holding down but now I just used #8 machine bolts to bolt a piece of 1/4 mahogany plywood into the opening. Started at the back and worked forward. Mahogany took double curve with coaxing plus I steam ironed as I went along. In hindsight I discovered there was 1/8 inch available which would have curved easier. At the front I found it was a tough go and maybe strut curves were out a bit so I replaced front 2 struts with one in the middle of where they would have gone. I made the front header from would so routed a rabbet in it for the plywood to fit into. This is an amateur job I admit but I am on a low budget project. Plan is to smooth out and cover with material .Some headroom loss with the flat interior surface but I have no chop.