[EricH]

Float planning already? Or are you doing something else?

I'd suggest taking a box of, say, wood (material doesn't matter right now) and lining it with, say, a trash bag. Now, the trick is to get a flat flow, so try cutting a flat rectangle out near the top of one side. Fill it with water from above to the point where the water is flowing out from the rectangle. Hopefully, the water flowing out will be fairly flat, and the box angle can be adjusted for the flow you want.

All the flat waterfalls I've seen have had a flat lip at the top, just under the water. As long as nothing is in the way, it should stay flat. (And, if you are building this for a float, that includes in the way in case of sudden start or stop.)