Using the material of choice, of sufficient dimensions (which may require some gluing up), drill the 1" holes where you want the rails to go. Perpendicular to the rail openings drill holes in each corner for bolts that will ultimately hold the whole fitting together. Then cut orthogonally through the 1" holes. Bolt the whole mess together.

If you use stainless steel you will need a machine shop with some nifty stuff in it, but I expect a high-tensile plastic would do the job nicely and you can do all the work with small power tools. Unless you are really steady you'll want something with a fence to make the cuts through the rail holes. You can rig something clamped to the baseplate of a jig saw or circular saw but a high school or tech school and some schmoozing might be your best bet. A shipwright or carpenter would probably do the whole thing for you in exchange for a six-pack.