I am restoring a 68 GTO convertible. After searching grave yards and after market extensively, I can say no one makes an aftermarket convertible wheelhouse but the hard top is similar. I found one in a grave yard but the guy wanted to rape me. (he knew what he had) I had to modify the hardtop wheelhouse to fit the convertible. The biggest difference is the top of the wheelhouse was flattened to accommodate the convertible top when folded into the body. I was fortunate to have one partially rusted original inner and outer wheelhouse and had to make a mirror image to fit the opposite side which was trashed before I bought the car. It took me many months and lots of trial and error fits to get it correct. Then I restored the rusted out sections of the original wheel house. Learned a lot about working with sheet metal and welding which was good experience when I got to the body and floor damage.
If you don't have a model wheelhouse to work with, I cant imagine how you can possibly fabricate one that will fit. I would find someone with a convertible that will let you spend a day making measurements and templates.