Filling the gap: I've used tissue paper mixed with (yellow/white) glue to make a stiff paste. It can easily be shaped when wet, but might shrink slightly as it dries if you glop a lot of it on (remember, these glues "cure" from the outside in).
This makes me think that a variation of this might be an untapered, plain BT-80. An uncut piece, 24 or 36 inches long, with two or three "booster pods" using rear ejection. Might need a touch of nose weight, which could be done with a wrap of solid core solder around the top lip. I've got absolutely no money to buy new parts, but I've got a supply of parts not used yet. I think I'll dig through the pile and see what I can come up with. A variation of this variation could have the pods "inset" into the bigger body tube by about half their diameter. Cutting the correct profile into this tube would be tricky, however.
Update: searched through the boxes of parts. Found a 14" piece of BT-80 - would this be long enough? Also found two matching nosecones in BT-60 (only two that matched, lots of odds and ends). The nosecones are elliptical rather than pointed (almost egg shaped). Last question: with rear ejection from a body tube this large, should I use just a plain centering ring at the top end, or should I add a (full/partial) coupler to help form more of a seal? Was also thinking of going full-blown stupid and making it either ring fin or adding an outer ring to fin tips.