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.

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.


The diameter of the starter hole is not critical - anything 1/8" to 3/16" or so will do the job. Since I was only drilling these 1/4" or so deep into the 3/4" plywood of the manifold blank, I used a cordless drill for this part of the process, being reasonably careful to keep the drill at 90 degrees.
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.

All of the big holes were then drilled (or is it "sawed" since a hole saw is used?) halfway through the double-stacked assembly. The center bit drills completely through both pieces in the process. Even with the relief hole, you need to go fairly slowly so as not to overheat the saw. Next itme I believe I'll make the perimeter relief hole larger.
Today's work ended with the manifolds about half done. Tomorrow I hope to flip the assembly over and finish the large hole in the bottom half of the temporary sandwich.
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.