I keep wondering if I'm doing something wrong.
You're not, and this is a common question when someone is looking at options for mounting a single tank to a backplate. Essentially backplates were initially designed for doubles, and there are four options for mounting a single tank to a backplate, and all with their own pros and cons.
A) Tank mounted directly to the backplate using cam bands threaded through the backplate slots, with the wing just sandwiched in between with no additional hardware. This keeps the tank as close as possible to your body, and as long as the cam bands are tight, you're mounting a cylinder to a flat plate and you don't get much roll once cinched down. There are only two downsides - if the wing has band slots much larger than the 2" bands (or open webbing like many dive rite wings), then you might need to adjust wing position to your liking each time you change tanks, also, if your wing has holes in it protected by grommets, now your cylinder isn't sitting on the flat of the backplate, its teetering on top of those grommets a bit and you'll get a little tank roll even if your cam bands are very tight.
B) Tank mounted directly to the backplate using cam bands threaded through the backplate slots, wing sandwiched in between, and book screws holding the wing in a fixed position to the backplate. This keeps the tank relatively close as well, but exacerbates the teetering tank issue since now your tank is perched on top of two screw heads. In this case you would definitely want to use plastic book screws to avoid tank wear. Slightly less ideal from a stability perspective, but at least you're not having to monkey around with wing position adjustments when you're doing a tank change on a rocking boat.
C) Tank mounted directly to the backplate as two options above, cam bands still threaded through the backplate, but some type of two-rod stablizer sandwiched between the tank and the wing, like that OMS stabilizer addon in a post above, or designed into a wing like these:
This slightly moves the tank a little further away from you, but not nearly as much as a metal single tank adapter, and your wing position can still be locked in place with book screws since the stabilizer not only helps mitigate tank roll since now the cylinder is cradled between two rods instead of perching on the center of the backplate or mounting hardware, but it also ideally moves the tank away from the plate just enough that you're not constantly grinding the tank on the book screws.
D) Single tank adapter - Instead of running the cam bands through the backplate, a single tank adapter bolts to the backplate, and the cam bands run through the STA. The cylinder is cradled between the two sides of the STA, so it is super stable, but it is extra hardware and moves the tank that much farther away from the diver. In reality, I'm not sure any of us would notice the difference of a half inch between an STA and no STA.
Although now you've got a stainless steel bracket touching your tank directly, and if you dive galvanized steel tanks I wouldn't even give that another thought, but if you're using an aluminum tank with fancy paintjob, you'll get some wear over time, and thats unavoidable.
Personally, the single tank steel plate I use for local freshwater diving has plastic book screws holding the wing in place and tank stabilizers built into the wing. I like that option the best, but there are no wrong answers as long as your tank is firmly attached to your plate.
