Good ideas, Al. If I was travelling in a more vertical environment I would want a set of shafts, to stop being over-run. I found the aluminum flats very easy to work with for backing-up the bolts. Your other point: that's another reason why I went with 2 of them -- so I could single-stack. I think it's easier to pull 2 sleds that are lightly-loaded, than 1 heavy one. And thanks for the canoe-trip wishes -- I retire Aug 01 this year, so hope to have more time for canoeing!
The sleds need to have a containment system. This keeps your gear together, negates the requirement for packs, and prevents snow from accumulating on your load during the day. It should be simple, strong, lightweight, and allow access to your load at a rest stop.
On the ski-sleds I sewed canvas. It was very tough, but needlessly heavy. I'd use nylon now. For the Pelicans I chose a simple 6 x 8' tarp. The tarp also serves as an emergency shelter. Very simple fold-over.
Here we are in the train getting organized for the drop-off.

To hold it in place I use a web of crossover straps. I DON'T USE, OR LIKE, BUNGEE CORDS! (Bushmen know a few knots.) This system works quite well.

The line in the right of the photo holds the hooks (store-bought). It is fixed in length and never is adjusted. There is a hook about every 16" of the sled. The line on the left is secured at one end, then goes up over the load and engages all the hooks, and is tied-off at the other end.
In use on the trail you can untie that end, free-up some slack, and access your load very quickly if required.

On the ski-sleds I used the same system, but made my own hooks from sections of copper brake tubing, pounded flat and bent around.