It's a Hail Mary, and gives you a string bikini look. You have been warned.

If you haven't tried the "bigger" method, do that one first. Support-wise, it'll be much better. You shouldn't end up with a tear in the middle if you're careful with the scissors.
If you're getting gaps through the crotch, though, then you may be having the same problem I always have. For me, it's not a matter of support. Rather, as a large male, there's just a bit too much of me down there for the narrow crotch of GoodNites and UnderJams to keep in. Pointing down exacerbates this by pushing the front of the diaper outward, while pointing up carries the risk that urine will trickle down around the outside of the leak guards, fill up that space, and then leak out. Basically, the narrow crotch is the root of all evil. The crotch width of your average size 6 baby diaper is 40% greater. Not surprisingly, I find Pampers baby diapers much more usable than GoodNites, despite needing to extend the tabs.