I'd probably do some kind of formed metal box to bolt to the inner fender.
If metal working doesn't work for you, another option, once you have a carboard or poster board shape to fit, is to cover it with clear packing tape then layup a couple layers of light fiberglass on or in it. The packing tape will mainly release cleanly when you tear the box out of the fiberglass, then you can do some filling, sanding, paint and done.
as an example, here is the mold for the bottom half of my glove box, before packing tape.
I wanted the box as deep as possible and had to work around heater box.
Masking tape will become part of the fiberglass, don't use masking tape as the final layer.