These are not DIY but are excellent: Neverlost Gear - Neverlost Gear - Best Kayak Paddle Leashes
I only have a tether on my paddle. It's extremely unlikely I will flip when I am fishing because I don't fish in class 2+ whitewater, no. Mostly lakes and mild rivers. When I do fool around on a fun river with rapids, I'm there for the ride and don't fish, and plan for possible flipping (dry bag, nothing valuable on board loose, PFD for the dog). A Nucanoe F12 is like, the aircraft carrier of kayaks, not too nimble in the fast stuff, and heavy. When I want to play, I take a real canoe.