Ever worked with fibreglass John?
It would be fairly straight forward to make moulds for hand laying the corner and 45 degree sections.
The biggest advantage of using fibreglass for the corner pieces, other than just looks, is that there would be very little difference in the expansion rates between the sheets and the corners.
You would simply glue these on, as you would aluminium edging.
I should also add that doing rounded 45's and using kerf cuts does not have to weaken anything. If you put glue in the kerfs prior to bending it will make the corner very strong. You could also glass over the inner side, hiding the kerfs cuts completely and making it waterproof too. Depending on the core material of the composite sheets, it may be necessary to use epoxy for this, as some of these cores will "melt" if you use an MEK based resin, but either way you could make a very nicely shaped box that way.