A few years ago I posted this: http://photo.net/large-format-photography-forum/00RMar
and this: http://www.apug.org/forums/forum37/5...tml#post710121
And when I first saw the eBay listing I sent the guy a message with the link saying I was very interested in his work.
The most likely issue I see with the current design is that as the unit turns in fluid it will generate a lot of drag on the edges, especially the corners. The Phototherm design compensates by having a support bar there, as well as the fact that the film curl on the Phototherm places the film in a better circle so that the leading edge acts in a more knife-like fashion rather than a hydroplane.
One issue Photohterm warns of with 120 film is that the rotating action of the processor can pull the film out of the grooves, so they have a little clip that holds the film tail in place. (Works great BTW.)
The Phototherm 4x5 adapter handles this with the plastic frame bar that holds the leading edge of the film as it turns in the fluid.
The current design on eBay - since it's not designed for rotary processing - doesn't take this into account. I expect that the film will come loose in the tank unless the rotation is pretty slow. I doubt it would take much more than two bands around the top and bottom edge to fix the problem (if it even is a problem), but that's more material and machining driving the cost up.
I approached Phototherm a few years ago about producing their adapters for the hobby market. While they were interested, the final decision was that it wasn't profitable. They even offered to rent the molds to me so I could produce them myself, but after crunching numbers I didn't pursue it either. A production run large enough to get the cost down to my target of < 20USD, so that they could be sold for 25USD was around 10K units. Smaller production runs cost more per unit, obviously.
Maybe the market has changed, and a higher production cost is feasible. And there's no harm in the fact that the guy on eBay has twice the volume capacity of the Phototherm unit. (6 sheets vs Phototherm's 4 sheets)
As far as his plan to a 5x7 unit, I've done quite a bit of measuring, and you can easily interleave 2 sheets of 5x7 into the same volume. As for 8x10, an adapter that's properly made could fit two sheets of 8x10 into a Paterson 5 reel tank. But by then you're talking about a lot of fluid to slosh around. But one made for a Phototherm could do 2 sheets of 8x10 in an SSK8 tank.