Should note that the wheel opening is where I marked, not just what I started to cut out. I previously sent pics to a couple people of the "finished" project and got replies asking if I was trying to convert them to straight skis and just how many beers had it taken to complete the project!
The hoop on the back is as cubdriver noted, just a raised bridge to keep from having the additional drag in the snow if it was a straight axle. With the back deck covered, the need is eliminated.
With the spring snow, it definitely builds up now on the back deck, but falls away pretty quick in the air, compared to the front of the ski buildup which makes a hard brick. I suppose there would be the potential to freeze up in the wheel well area and prevent ski retraction. I have had other portions of the ski hang up and have found that simply touching the tails to the snow as I fly past a ridge on the way home seems to free things up nicely. Historically this is the event that my passengers seem to recall with the most excitement in their retelling of a fun day ski flying! "And then he says 'we're just going to bounce the skis off this mountain top to unstick them'!"
The main thing I was/am concerned with is the potential for the tail to act more as a shovel as opposed to flotation. I beveled the leading edge of the UHMW behind the tire, where the back deck starts, but obviously this will have minimal benefit to keep the platform floating upward. I thought about putting a doubler on the leading edge to help it bow up and keep it from bowing down and digging. After a couple days use however, the leading edge is bowing up and forming an upward arch on its own.
The 1/8" UHMW cuts relatively easily and if there is a problem, I can simply remove the back deck, even in the field, and start over.
I did talk to one guy who had been willing to be a test pilot and did the mod to one ski and the other was with the open back deck. He said he noticed a bit better flotation, and smaller turning radius with the closed deck on the inside (reckoned the open rear gave more rail effect and harder to turn) but when he did the comparison test, the snow was pretty compacted below and not more than a few feet of fluff on top. Surprisingly there was no yaw or roll effect from the assymetric surface of the skis, but he noted that as with external loads, you wouldn't want to do stall/spin testing like that! :^o