The rapala tough screen is what most guys use. I think they are around $40. Weigh your fish lock the weight in and touch slot 1-8. When you get 7 you just replace the smallest fish by touching it's number. Really simple. When I purchased mine I thought it would be a lot more money but I love it.
Academy has culling floats. The ones I've seen are big floats for bass and too big for crappie IMO. Academy also has a culling system with colored rubber tubes and a small dry erase card. The tubes have small clamps on the end. I don't like the clamps as they tend to come lose. If you don't use the Rapala system, you'll need a small dry erase board and marker to keep the weight written down. Or a small pad and pen for numbering fish. The dry erase system is a problem in the rain and I had to keep mine in a zip lock bag.
The Rapala system works better. Most are accurate within a few hundredths of a certified scale. Also has eight small red floats that attach to the fish to correspond to the number on the screen. Helps find that smallest fish when you need to cull. The clamps that were attached to the float didn't last long and I replaced those with brass safety pins. Those will stay attached to the fish. The small red floats are just the right size and don't hamper the fish's ability to swim in the live well. Keep some extra AA batteries in the boat for the scale. And there's eight markers for eight fish in case one dies.
Another good scale is made by America Weigh Scale and available on line. Another good accurate scale. Models are SR-5 and SR-20 and run on AAA batteries.