[shilala]

Mahogany plywood is a veneer. It'll generally be a thin slice of mahogany over a pine or birch core. No plywood is any good for building any part of a humidor.
Any type of non-aromatic lumber is okay for building the actual humidor, but Spanish Cedar is used as an aromatic lining to impart flavor to cigars. That's non-negotiable and not a place where you can cheap out.
Good humidors cost a lot of money. The materials and hardware cost a lot of money. Buy, by far, the biggest cost is the time that goes into building them. If you have the skill to build a humidor, there's no amount of money you can pay for material that does not leave you at a great big bargain.
Shop a little, you'll find Spanish cedar at a reasonable price. I use Goosebay Lumber.