Just been down to Screwfix and now have a bunch of 60mm M6 bolts, wing nuts, and mending plates. I'm essentially making the Stellafane primary mirror mount, though rather than fitting inside the tube and screwed in from the sides, this will be mounted on the back with L-shaped brackets secured a couple of inches up the sides. Will probably be a 4-point mount, with three sprung collimation bolts and lock bolts.

Short of drilling the inner ventilation hole(s), sanding, sealing and painting, and marking/drilling the glue dimples, I now have a Stellafane mirror cell top part.

http://stellafane.org/tm/dob/ota/cell2.html

I still need to determine where the glue dimples should go. Stellafane recommends 71% of the diameter. I've heard from someone else it should be 40%. About 2/3 of the way down the page here (http://www.atmsite.org/contrib/Holm/Plop_optimized_cells/), it quotes deformation values for both (based on an 8 inch, 1.1inch thick f/8 pyrex primary). It appears that 40% is correct (Thanks, Chris!)

The mirror cell bottom part is next. This is where it gets ugly.

I have 12-segment tubes (hardboard + fibreglass/resin). The strength of these is unknown, and the weakest point is no doubt the longitudinal butt joints between the segments. The Stellafane mirror cell bottom part has three points, which sit inside the tube and would be screwed from the outside. I've got a rib right at the bottom so I can't put anything inside the tube - it's going to have to go on the back. I've got mending plates (75mm long) that I intend to bend into an L-shape roughly 1:3. The short part will support the bottom of the cell and the long part will attach an inch or so up the side wall of the tube.

To hang the mirror from three tube segments will put shear force on the butt join - I know I'm not going to pull the thing apart with a few kilos hanging off it like this but it's making me think. Would a 4 or 6 point bottom part be better, to distribute the load across a greater number of tube segments? I start getting into difficulties balancing the need to have some meat for placing the collimation and lock bolts and trying to remove material for ventilation and wight reduction. I think I'll stick with three, and where I screw/bolt into the side wall, add internal bracing in the form of 3 small internal blocks, each secured to the respective adjacent tube segments.

I'm also half considering modifying the bottom part. Rather than using a single 3/4 thickness of ply, using two 3/8th thicknesses of slightly different sizes. The internal diameter (round) of the ribs at the bottom is about 1/2-3/4 of an inch less than the outer (12-segment) diameter of the tube itself. By having one thickness of ply cut to fit that internal diameter and the second thickness extending all the way out to the side to accept the bracket, I'll move the weight of the mirror 3/8" further up the tube and I think it would improve the aesthetics slightly as well. I want the weight forward as it's a three tube double truss construction and as the middle tube goes at the center of mass, I don't want the bottom tubes to be too close to each other. 3/8" isn't going to make much difference, but I've got more 3/8" ply than 3/4". ;)