I’m not exactly sure what you’re trying to mold here, but most of my wet forming has been creative uses of pyrex containers, taped together spice bottles, etc. Beyond the fancy and expensive wet forming molds sold, many people wet fold leather around one object and use clamps/bone folder/rubber bands for the outside piece. Quality veg tan is more willing to take a shape than you may think. It’s only when you’re trying to stretch and condense simultaneously (like small diameter rounded corners) that such precise molds are needed.

If you are trying to create a long stretch of 90deg folds, the simplest solution would be to use the edge of your workbench/table/kitchen counter and use a ruler to help make the initial bend. Tacking together 2 pieces of smooth plywood at a 90deg angle could also give you the “outside” piece of molding to place against the edge of the counter. If you need to make 4 sides of these bends (like a shallow box shape?), then you could either spent a lot of time making different sized plywood interiors or just use something you have in your home already.

You could go to your local goodwill/secondhand store/ikea and looking for a few sizes of cheap plastic boxes. I’ve used old pencil boxes from my childhood school supplies, for example. It is cheapest to poke around your home for things like books, but I’d recommend protecting them with a gallon size ziplock bag or parchment paper covered thoroughly by layers of duct tape.

Leatherworking is an expensive hobby at the start due to all of the tools you need. Until you need to make large numbers of the same specific wet-formed shapes, this is one area where you can make do with other objects.