Okay Jess I just got done binge watching all of your videos. I got all the way to 24 last night, well at 345 Am.
Couple of things, on my old tnttt posts I talked about gluing and riveting my SIPs to a frame. I like your method a lot, it is just a more of a complete "system" than what I had in my mind 8 years ago. So great job on that, much better than what I would have done.
I was thinking about giving you an idea for when you had to bolt thru a panel and needed compressive strength. You came up with something, but there is a better and easy way. Say you need to run a 1/4 bolt thru the panel. Drill a 1/2 inch hole in the top layer, then take a stiff wire with a 90 degree bend on it that is about 3/8s long. Chuck the wire in your drill and use it to ream out the foam a 3/8s larger than the 1/2 hole. Then fill the hole with epoxy, this will give you a large solid surface to bolt to, including a washer if you want it. Just drill thru the epoxy where you want you final hole to be. That is how it done on foam core boat panels, as it give the compressive strength as well as guarantees water tightness when sealed. It is kind of a hold over from balsa core days, where water in the core is very very bad. In your situation you could also drill the hole 3/8 for a 1/4 bolt and have some wiggle room for adjustment or to give a panel the ability to shift and slide little for thermal expansion or torquing on the frame.
As for the epoxy no gripping to the pvc, yup you learned that one. What you could have done pretty cheaply these days if epoxy reinforced paper tubes, or carbon fiber tubes ( thin wall ). This would have no taken away the strength from the panel as the round shape would actually probably be stronger than the foam. For bends just a miter cut for every 10 or 15 degrees and super glue to hold it together. It would have been very easy to cut the foam with a round router bit to have a nice tight glue up.
carbon fiber tube amazon link, but I would find a thinner wall for cost reasons. Imagine carbon fiber arrows but a larger diameter.
Amazon.com: ARRIS 16mm x 18mm x 500mm 3K Roll Wrapped 100% Pure Carbon Fiber Tube (2PCS) Glossy Surface … : Industrial & Scientific
www.amazon.com