Reply to SJ:
I looked in a Cat One Safe Source parts & accessory catalog but didn,t see any locking caps in it. They are a big diameter cap as you all know so would be expensive to buy one if they were available so I don,t have an answer for the problem except the device that Rich mentioned. The problem would be to catch them.
In the last fuel crunch about 1978 or so, I had to put a chain around my sprayer and padlock the engine covers down. It held 35 gallons of gasoline and was parked in an unwatched place.
A neighbor opted for a locking cap; they just punched a hole in his tank, let what they couldn't catch run out on the ground. Lucky no smokers came by next morning. I decided against a locking cap.
However, the locking cap on my fuel storage tank is a big bar hinged to the cap, the other end padlocked to a steel vent pipe that is close enough on top the tank. When locked neither the cap nor the vent pipe can be turned. I'm presently not storing diesel in my high tank, using the portable fueler. It's too easy to drain a high tank.
Good luck and good shootin'.
Jack