I'm sure most of you know by watching the pro's doing it, and that is using a culling beam. Basically a rod suspended in the middle by a string. With two hook on each end of the rod to put the fish on. The bigger fish go down on the scale and the one you keep. A culling system is not needed in DABL and I'll leave it up to the team.
You could probably make one yourself and have seen some. But there are ones on the market that are reasonbly cheap and built well. You can check out most major tackle retailers for them. The other part of a culling system is marking the fish. Basically you need to know which is the smallest fish in your live well. So the next fish you catch you can weight it against it. All systems have some kind of marker to put on the fish, usually a clip (with a numbered tag) on mouth of the fish. There are systems that you need a pen paper all the way up to fully electronic ones.
The last pic of the green grip and weigh scale is slick. I have this one and we'll be using this scale to weigh your fish. DABL will most likely get one for the league, just in case I can't make it.