You've all hit upon critical details, all of which lead me to believe the plastic pipe prototype is likely the best next step - Intrepid & ajoh, by some stroke of luck the T3 is about as dialed in for preformance street handling appropriate for it's scale & moments as I had any right to expect, better then I'd hoped for... should I change anything I
might be able to compensate but likely not & a cyclecar deserves it's own dedicated build in any case. They're not particularly complicated which is ofc part of their charm. The only reason I mulled over the idea of a chop convert is absolutely all of the main components that comprise the T3 would be about perfect for the cyclecar I'd like to build (the unholy crossbreed of vintage appearance & modern materials), tested & true I'd need reinvest in quite the shopping list... while the T3 was an economical build for what it is, my budget is still enjoying the break from what seemed to be a never ending endeavor lol, perhaps next season?
silverbear & cannonball2, aside from I'd never subject the T3 to the winter road conditions enclosed or not (corrosion) - you've also hit on concerns leading to the possible prototype, one of the critical features it has to have is a "convertible" aspect, not just a canopy but the entire shell must be at least somewhat easily removable as well. Not only do I like "ridin' nekkid" (the taddy, not me, sunburn lol) but in order to get the T3 into & out of my shop space I have to tip it on it's side & wiggle it through, with a veloshell this wouldn't work as I've about a half inch to spare as it is. A bit annoying I still very rarely leave it parked out in the elements regardless - with a velo I'll throw together a small shelter outside for it... but I'd still like to easily get it into my shop, or jus' run around "nekkid" as it were.
Using the light aircraft materials is very appealing, I've some experience with
Dacron & the UV deterrents have come a long way, but that & the
ultralight geodesic design I'd like to use while comparatively simple they're yet still a bit more of a commitment then I'd like to make at this stage... the cheapo mock up will serve to help me ascertain better the shapes I need rather then like, modifications & alterations simpler & cheaper for the inevitable do-overs I've in store. I figure one of two possible results - it'll be subject to structural inadequacies due to the stresses of the speeds I'm traveling (frm just flutter to catastrophic failure/collapse) or it'll work so well enough it'll be then refined, to be kept as-is or replicated with the materials suggested.
fasteddy - a very good question, I'm not sure as that depends only on the rigidity of the internal support structure, of which depends entirely on my design ability. I'm fairly confident I can make it stable enough, the issue being will it be stable enough all the time or will there be an undue amount of inherent flex regardless of supports & gussets. You can ofc have two separated layers of shrink wrap, that's a bit tricky though as is any internal filler - it can't touch anything while hot as they layers will weld together/distort from cool contact. If there's any flex at all in the shared support, they'll never quite balance out, you'll be chasing wrinkles till you've a meltdown. You can however reshrink later after it's cooled or any time it jus' needs a snugging up & now that you've mentioned it I'll likely add a interior covering, if only for aesthetics.
Good thoughts all & I thank you, it helps me rethink stuff I thought I had sorted (but maybe not lol) as well as gives me new aspects to consider
Thanks ajoh, it's defo cool & thanks for the link! Pity it's a few years too late for my project, had it been available the build might have taken a different path entirely - although in retrospect I suspect I may be better off with the hybrid aspect, insofar as engine reliability & longevity is concerned 
