Hi Matt,
I made my own and it was no big deal if you have the standard panel blank as a template. I bet you can find a bending brake somewhere nearby if you go look.
Steps are:
1) get yourself some right-size sheet ali.
2) bend the bottom flange
3) position and clamp the standard panel blank over the sheet.
4) match drill and cleco the rivet holes around the top.
5) mark out the outline
6a) remove the template and cut around the marking, or
6b) as I did, course cut first then use a router to match cut the template shape.
I cut pretty much to the same shape as the standard but if I were doing it again I would try to include the underhang below the sidedecks where the airvents go in the panel shape.
Or the simpler alternative is to call up Van's and ask the to make you a deeper panel, which I believe they are more than happy to do
Best of luck,
Steve