TechWriter,
I guess the question is...how many more years do you expect to use the rig?
The marine grade plywood will probably last 10+ years in that application. Probably longer if you treat it with preservative before it's installed. While it will get wet with road spray, it should also be able to dry. Aluminum or plastic would be permanent...but as you said, there's a price for that.
I know if this was me trying to make the decision, I'd probably drive myself nuts trying to decide.
If I go with the less expensive plywood option, and then the work done on the back looks A1 perfect, I'd be kicking myself for not using aluminum or plastic. On the other hand, if I put the expensive stuff behind it, and then the back didn't look so good, I'd be kicking myself for spending more than needed.
Knowing myself....(after 60 years, I almost do! LOL)...I'd still probably opt for the marine grade plywood option. And SHOULD the back come out better than expected, I'd just spray some wood sealer up there every couple of years...if it even needed it.
Well, sounds like you've been assembling a great diversity of advice & opinions and sorting through it all to find a good solution for your needs. I wish you good luck with this project, and I can't wait to see how it turns out!
~Rick