In order to be able to measure the total end-to-end dimension of the assembled collector tubes, including the manifolds on either end, I began the process by creating the two manifold pieces themselves.
First, a test hole was made in a piece of scrap in order to see how the ends of the cans would seat in the 2 1/8" hole. Why this diameter? It seemed the best fit of the hole saws I had on hand.
Next, making allowances for the 3/4" insulation board the cans will lay on, I laid out a line showing where the top edge of the top of the can would end up. Then, dropping down 1 1/16" from this line, I drew another line representing the centerline of the 2 1/8" holes that would create the manifold.
I don't know if this picture helps you, but it will help me remember how I did this if I build another of these as I hope to do.
It might not show up well for you but this shows, on the top of the left-hand board, that allowance was made for 3/4" of insulation, this time along what will be the side of the tube tray. This will all be clearer when you see the tubes actually installed in the tray.The holes along the centerline are spaced at 67mm on center as recommended by one of the many sites I consulted on the net. I used a 5/32" brad-point bit to make these starter holes. This bit allows accurate placement of the hole's center without the bit veering off the mark as it bites into the wood. The larger center bit of the hole saw will then follow this hole as it starts, staying on center as well.
Above shows the two manifold blanks temporarily held together with double-sided carpet tape and ready for the drill press and 2 1/8" hole saw to do their part.
Taping the parts together allows drilling both pieces using only one layout.
Next, shallow passes were made at each hole to show the outline so that a smaller sawdust relief hole could be made at the perimeter to help keep the holesaw from overheating.This relief hole can be seen in the last photo in this sequence below.
I'm thinking once these manifolds are done, the frame will come together fairly quickly and you can finally begin to see where we're headed.
However, I reserve the right to obsess and over-engineer as the mood strikes.
As always, thanks for looking.