Last year I spun a prop and vowed never again to be without a spare. But I didn't want to loose any space in my compartments. Once I got some BR input, here is what I devised:

The first picture shows the battery compartment of the boat. I took a SS 7" X 3/8" bolt, ran it through the prop hub and then thru the side of a storage compartment. I used a wing nut and fender washers to secure it. The prop is laying on the bottom of the boat the bolt just holds it secure.

The second pic shows the inside of the compartment. You can see the bolt head/washer on the right. In the bottom of the compartment is my prop wrench and a plano with tools/parts needed to change out the prop and make other minor repairs out on the water.

that looks like a nice idea.You may also wanna keep a spark plug wrench and extra plugs in a box if you can.