Kevin, glad to see you're back at it.
Plexi-glass? Wow! Can't be cheap, and that ceiling layout, with all that copper and tubing, is really famous. Musta cost many thousands to build it throughout the house. I doubt if you'll use it, but if you do, please give us all the intimate details.
Kevin and Will, I still recommend the solid 3/4 inch pine. A great deal more cost-effective. As I told you, I sanded and sealed each board three times. It's been up there five years now without a budge. The doweling helps and none of them are glued except those three in the yard.
Here is a close-up of the nice smooth edge you can achieve. IMO, gluing an edge to plywood is gonna be a real hassle, especially in the corners.
Here is an example of three 8 X 8's glued, doweled, and clamped together for my small yard. The ends are supported as shown. There is only one small bracket holding up the middle. I worried about this, but decided to give it a whirl.
Just took this pic today. As you can see, no warp, no bending, perfectly straight after five years.
You won't see the top side, just the bottom.
The bridge looks great. At 8 1/2 inches, you have an inch more than I do up there. Should be plenty.
Your pics of testing the double-track for the proper spacing, reminded me of this. That Army engine has a wide butt!
I hope this helps. Good luck and please keep us posted on your progress.
Jerry
