Makeshift rulers are not a new concept I'm sure but I found that the little dimples on Malibu hatches line up pretty good and can serve as a nice ballpark ruler. I painted some of the dots, 1 dot (11"), 2 dot (12"), etc. Nice for a quick OTW measure for a legal bass and beyond. My hatch is good up to 15 1/2".
Just bring a tape for the bigger ones