I used to fish a C&R tournament where they gave you a measuring board and a disposable camera to photo the fish you caught. The "board" was basically a vertical blind slat with a yardstick sticker on it. Longest fish regardless of girth. These were hardier fish (redfish) so handling out of water was not a big issue.

I have also seen C&R tournament where they make fish cradles that have measuring sticks on them. Fish are measured and photo'd without removing them from the water with longest fish winning.

You might want to make it a pairs event so that you have at least one first-person verification other than the angler's.

Just a couple ideas for ya. My method of stuffing them into milk jugs is just not conservation minded...