I recently bought the jet harness plate and wing, and threaded a dive-rite harness through it. I've only had it in the pool so far, but it appears to be very well made and the price is right; about $250 for plate, wing, harness, cam straps.
I also have an issue with the regulator hitting the back of my head; I need to do a little more adjusting to get it right. If I can't get the vertical adjustment to fix the regulator issue, I'm going to make a couple of small spacers to fit where the cam straps go to get the tank a little off the plate; sort of like an STA. Probably this won't be necessary, and I really like how tight and secure the tank is on my back.
Regarding the shape, it's my understanding that the whole original purpose of backplates was to securely hold doubles; they were never intended for single tank diving. Am I wrong about that? Okay, so divers got used to the nice things about using a plate, and wanted to do so with single tanks as well, hence the STA. But, remember, the entire reason plate were shaped with the center channel was to mount doubles, correct? So, why wouldn't a flat plate be better for single tank use?
The main reason I got the plate was to get the trim and weighting benefits of a BP for recreational, single tank diving, especially for use with aluminum 80s, which is exactly the intended use of the jet harness stuff. I suspect he's not going out of business; there were a few plates and wings/straps on ebay last time I looked, although it does say that his store is closed. The stuff is listed as individual items on the BC category.