I would support the introduction of single machine sheds. I would use them; however, I would propose going in the opposite direction of no or lessened bonuses. Make them all expert bonus shops that use T6 construction resources to build instead of the t5 they are now, and maybe need more resources to build. Would be a bit of an overkill on a casting box, but if you want the convenience of a smaller footprint, you pay for it with t6 construction.