Tim,
The way I've drawn it, the width is the same as the height (in oven mode). Height and width wouldn't necessarily have to be the same, but your spokes would obviously have to be long enough to fit the longer of the two dimensions. I arbitrarily drew it at 12" x 12", but you could pick any dimension that balanced weight and utility, I'm sure. It looks like the longest spokes commonly available are a little under 12", so it could be that 11" or so is a more practical dimension. I figured one end of the spokes is already crimped, so that would keep it from coming through. Maybe the screw-on nipples that come with the spoke could be used to secure the other end…? Even titanium spokes don't appear to be too expensive, if you buy them in quantities of 25 or so.
As far as a shelter heater..I think you could adapt this to work for what you suggest. I'm not sold on wood-burning stoves for heat on a small shelter, though. I would be very worried about fire risk, oxygen depletion, carbon monoxide, etc. I think wood stoves work in big canvas, outfitter-style tents because canvas is breathable and not readily flammable, and those tents tend to be drafty, anyway. I know this is a discussion that has been touched on in other places on this forum. What little I know is that if your fire box is not 100% sealed from the living space, then you need plenty of ventilation so you don't have CO buildup or oxygen depletion. Having a drafty tent sort of negates having a stove, to some degree. The other option would be to have the firebox sealed completely, with intake and exhaust to the outside of the shelter in a closed loop, which is probably not practical with a cobbled-together stove.
I think a better option might be to build a fire in the reflector, with it tilted toward an open tarp configuration–similar to what the old-timers did with lean-tos and fire reflectors. Not as efficient or neat, maybe, but probably still useful, and safer.
Re: learning SketchUp–the Push/Pull tool is your friend. Once you learn how to use that well, everything gets easier.

