I use a very minimal “slope” on the rolling trays I built out of fence posts and 2x4
It’s been years but I’d say 1/8th inch is a good differential.
I’ve ran that NFT plastic fence post set up, they can be finicky, water temps etc.
Do yourself a favor and buy a pond liner and throw it down under your grow.
Water will be spilled.
Happy Growing
Gwern