Gluing plexi or something?? To pond shield??

[takagari]

So I have a 500 gallon plywood tank.
I installed 3 h2o overflows. Which I've come to realize.are.horrible for monster fish....

I want to remove them and install an overflow box.

As I have 3 1.5 inch bulk heads near the top. I'm wondering how do do this?
I'd thought of making a plywood box. Pond shield that. Then silicone it in?
Then an inch of egg.crate at.top?

Any.other ideas?

Thanks