How about making it out of plywood? You could glue and screw (or air nail with a brad gun), fill any defects, sand, and finish with a high gloss finish. A couple of sandings between coats of finish and you should be able to create a very smooth surface.
You might want to weight the box before closing it up so it isn't excessively bouyant while making the mold.
Do you want to mold a smooth-surfaced right rectangular prism or just a rough block for further work? If you just want a block mold, visit a commercial cooking supply store and look for the appropriate size baking pan or mold. Some bread pans are about the right size and they even have plenty of draft for unmolding.
If you want a straight-sided block you might be able to find a suitable plastic storage box in an office or home supply store. It should still have a wee bit of draft (a good thing) because it had to come out of its mold. Fill it with sand to help it hold shape while you cast your mold around it.