Hi Robert,
I made the ones for Harvey's T2 from mild steel plate, and used the sizes that Richard quotes in the build manual.

I first made them from aluminium, but couldn't find anyone who could weld aluminium (to weld the edge pieces on), so I re-made them all from 18 swg steel plate.
I cut the back bit (where the lights mount), and folded the edges over, as you have done. Then I made the small 'rounded over' section (for the body curve) by first cutting a strip about 4 inches long and the same width as the edge flange, and bending the metal to 90 degrees about halfway along.
I then forming it around a piece of 19 mm solid bar on one side of the fold with a hammer, until it was the right shape.
I then cut the piece down to fit the cap in the edge flange, and had Gary T.I.G. weld it in place.
If you make them from a flat strip, it doesn't matter which side of the fold you curve the metal, as they are then reversable.
Once they were welded, a little grinding to tidy them up, car body filler to smooth out any imperfections, and a coat of Silver paint to finish them off.
Before final fixing, I marked out the light hole positions, drilled them out and filed them smooth.
I fixed them to the car with 'Evo-stick' and 2.5 mm 'pop' rivets to add the touch of realism.

It sounds complex how I have described it, but it's not really. I hope it has helped you out with your problem.
Keep Busy,
Regards,
Mark.