Storage in the sheath isn't all bad. A properly fitted sheath keeps the edge out of contact with everything, and as for moisture, you should never allow your blade to be without a fine coat of oil anyway. How long are you going to keep it stored? In any event, make sure that you keep a very fine coat of oil on the blade, and keep it in its sheath when not in use. You could get a shirasaya made, but keep in mind that the practical plus is the low end of forged katanas. The shirasaya would cost a lot more than the sword itself. DON"T leave the blade in open air, whatever you do.
If you want the ultimate in rust free storage solutions, get a vacuum sealer and some long bags for it. Take the blade down, wrap each component in newsprint or some such material, and vacuum seal each component. This is how some katana polishers store their stuff long-term.