Here is what I have available.

3/4 inch marine plywood. No tongue and groove at all.

a single piece of treated 2X4 10'is $20. so I will buy untreated and treat it myself. seeing untreated 2X4's run about $9
Treated 4X4's at 10' are almost $30. Again I will buy untreated and treat my own.

I was going to use epoxy in/on the joints and corners of the Plywood placed on a 2X4 joist and 2x4 X12" support walls. Overlap the bottom plate making it 4X4 and bolting in some 4X4's as posts with 2X4 angled for support from each post. doubling up the joist where the plywood comes together in the middle.

Then I will coat all corners where walls and bottom come together with Elastomeric rubberized coat with reinforced cloth once the corners are done I will lay cloth with a Elastomeric coating over the whole inside of the trough. Building up to 5-8mil thick. once dried I will coat it with white non-toxic swimming pool paint inside and out. That should be water tight.

Do you think that will work

Does anyone know of any youtube video's on how to build a "Swirl Filter"?