Lots to think about and great to have your input challenging my ideas, so I'll keep pushing my thoughts on here. It's what I get for making it up as I go along, and using new materials and processes all the while - fighting the learning curve.
What I'm (currently) planning to do, more or less in order...
- take a small hacksaw blade, stanley knife and files to each of the rib butt joints and clean them up.
- fill those gaps with a wee dob of some epoxy putty I have and shape/smooth it nicely after 5 minutes setting up - that will give me the continuous 360 degree internal structure that I don't yet have - just a few grams in total.
- re-mat/resin the back of each joint, maybe nestle a short length of bamboo skewer in behind each joint first, for fun - a few tens of grams more.
- skew-pin short finishing nails through each hardboard face into the existing ribs. That's just belt and braces, but only a few grams - why not.
- add at least one additional baffle/rib in the top tube anyway, just above the focuser hole, but this won't be 'doubling up'. Given the length of the middle tube (14") and the fact I'll likely be adding mounting rings to here, I should probably add 2 more internal ribs there, too. Budget 100g per rib, all in.
- chock out the external geometric profile on the middle tube, top and bottom ends of it, each with 4 double-thickness MDF triangles (36mm thick, 75mm wide - width of one side panel, just under 18mm tall) - basically extending the line of the panels either side of it to meet at a point, resulting in the irregular octagon seen in one of the pictures above but without the additional thickness all round - just the 4 points. Budget maybe 50-100g.
- fire a screw or two into these wedges from inside, lay a strip of mat over the top of them, extending off to the adjacent faces. Grams.
- fit similar wedges on the inside of the bottom end of the top tube and the top end of the bottom tube, to give meat for the threaded inserts. Budget another 50-100 grams
- make one full ring to terminate the trusses from the top tube, matching the irregular octagon shape of the top face of the middle tube - making the scope split in two. Provide a way of bolting the top half to the bottom half. Budget 250 grams.
- give it all a good hard sanding, trying not to break them.
- re-mat/resin the lot (probably fit the external triangles on the middle tubes at this point, while wet, giving a layer of wet mat under them - clamp and add screw when cured, probably unnecessary)
- drill for and fit the threaded inserts for truss attachment
- sand and paint
- design, build and fit mirror holders, focuser and finder
- weigh and calculate truss lengths
- cut and fit trusses
If I'd used a heavier mat in the first place, one coat would have been enough but the tissue is so thin and brittle that I wouldn't trust it under a knock to provide any more rigidity than the structural support afforded by the ribs. Another layer of 'proper' mat may well - I read some canoes are made from just two layers of 300gsm. So I think I'm set on the second layer of resin/mat. Budget around 750g per OTA if using 300gsm, but offset against removing up to 1.5kg in the external rings.
If I am set on another layer of mat/resin then, I'm going to have the rigidity I need in the round with minimal additional work needed on the internal ribs, and am happy I can add the meat needed for the threaded inserts. I think that's it - not going to need 'straps' of metal around the edges or anything like that, though I've seen folk do that and it can look really nice. I think I just need wood, mat and resin
Perhaps one could get a better finish with a bit more thought/patience/etc... but for a hybrid of the polygonal tube and Serrurier truss designs, it's getting on to be not bad at all I think.
Thanks for prompting me to ditch the rings. It's going to be a few weeks, yet...