The only change to your plan that I'd make is to make it more like a paint roller tray than have steep sides on all 4 sides. Having a sloped end will make it a LOT easier to seine and get out the forage fish. Make it longer than it is wide and you won't need that long of a net to seine it.
Good idea! I was mainly focused on the weed control aspect of things but having a gradual slope would make it a ton easier to seine it out.