Long before I actually started building this thing, I figured I could try to fabricate the transitions. If I couldn't come up with some decent ones, then no point in continuing. And really, it's the big one that is the most critical.
The template comes out to a bit under 9x14":
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Finding something officially labeled "card stock" at that size was fruitless, at least in any store I could find locally. Finally, following Gary's advice, I nabbed a pack of 14x22" poster board from Walmart. I think it was this, although I can't seem to find my pack of it right now:
https://www.walmart.com/ip/UCreate-Poster-Board-14-x-22-8pk/17203958. Of course there is no weight listed on the package (or online), so I just tried to gauge it by feel, and ensure that it was more than 65 lb. I really have no idea of its actual weight. It also has one shiny side, which I figure should go on the outside, but who knows.
For my first attempt, I cut the join tab as a separate piece and glued it to the inside. Failure. The stiffness of the two layers of poster board meant that the seam area wouldn't bend well. That one was so horrible I tossed it without taking a picture. But I learned.
So for the next one I just went with the standard overlapping tab. I followed Gary's strategy of rubber-banding the paper into a tight cone to put some curve in it before gluing:
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Then I had to figure out how to actually hold the thing in position while the glue was setting. I took two pieces of wood strip, rounded them off so there would be no sharp edges to dig in and mark the paper, and fashioned a crazy clamp setup:
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There is probably a simpler way to do it, but this arrangement worked pretty well, once I went through the juggling act of getting the wet-glued transition into the clamp. I Bondo'ed the seam (first time using Bondo), which came out "OK"; the seam will be visible on the finished model, but I'll live with that. After getting this one done, the other two transitions were straightforward.
This photo gives some perspective on the size of this transition:
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This was all done months ago, then my whole workbench got packed up and a major home remodeling project got underway, and I focused my efforts on finishing up Biohazard. But I had passed my major first test, and knew that this build could go forward... eventually.
Oh, one more detail: no centering rings have been installed in these transitions. This build will take a different approach. Because my BT80 extends all the way through the transition, I didn't need a 55/80 ring in the front of it (it'll just slip right over the BT80), and as for the back... well that will wait for the next update.