Cheapest way to lock up a non sure grip?

Yup. The buzz box is the cheapest way, hands down. Just do it so you can still remove the center section and you'll be fine. Don't be conservative with the welds and it'll hold up. I would weld the spider gears TO the side gears and the carrier everywhere I could get to them and weld the side gears TO the carrier everywhere I could get to them. The more welds, the stronger it is. As long as you can still get the axles out, you can get the center section out. You should disassemble the whole thing, wash it completely off as mentioned, reassemble it and weld away. People say "it will break eventually" but if you weld it like I described, the axles will break before anything. No it ain't the rightest thing to do, but it's the cheapest and that's what you asked. lol