I like the idea of an acrylic back window. You could even use it as the I/O shield for the ports and to hold the cards in place with bolts epoxied into place on the other side of the screw holes.
What I’m worried about is that bar that’s flapping in the breeze near the PCI slots – get some scrap metal (even an old I/O shield will do) and fix that gap either by arc welding or by riveting or even epoxy or nuts & bolts. It doesn’t need to be very strong, just strong enough to keep the thing from wiggling and add case stiffness so your window won’t pop out.
Hope that helps.