honestly, i use to fish from a kayak a lot for these contests and i found out pretty quick it was almost impossible to do safely so i stopped trying(guys who do it use a huge fold up bumper board that has raised edges and almost like a box, hold the fish with the ruler sticker on the inside of it, i'll post an example) if kayak fishing is your method, you'd have to make or pay big bucks for one of those big fish holding bumper boards. the one in the pic i found has a piece that attaches to make it longer if your fish is longer...it takes up a good amount of room though.
I do most of mine from a flat bottom jon boat, easy to have the ruler already out and (keep wet if hot in the sun) lay the fish down quick pic, selfie and back in it goes. with the striper in the example i'm in my friends saltwater boat and the ruler and fish are both pushed up against the cooler (straight edge) and i just lay the tip of the fish's tail on the measuring line so i can zoom in and see it's exact length. the best crappie is on a regular effective cheap bumper board you can buy almost anywhere that sells fishing stuff.
you can look on
www.onlinefishingtournament.com site to see more examples of good pictures and bad measurement pictures.
