I'm cool with that.
I used 1.5" rigid Styrofoam from home depot. I wanted to seperate the 2 sheets of plywood so it would be stiffer when i screwed it all together and let me tell you its very very stiff. I was going to add a 2x4 lip around the top but it doesn't need it and i like the rimless look.
Im really not sure how much pond shield i need so i got 1 1.5-quart kit in white. If i need more ill order it from amazon and it comes next day.
https://www.amazon.com/gp/product/B00D2ZWLGQ/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
Thanks! I want to do something very similar to this once I have some more free time.