Devil's advocate, one big **** up, something getting stolen, catching on fire, a sue happy dumbass, and all that extra hidden cash gets to pay for it because your insurance is going to say
Go legitimate, there is really low overhead; insurance (you need that on any business), you don't have employees, maybe a light bulb in each unit if you want to get fancy, biggest expense is a building. Say you build a building with 20 spaces 10'x10' and charge $75 a month for each, (on the low side) that's 18k a year. Have the building paid off in a year or two, then also charge for outside storage for cars boats campers ect, Fence and gate it and get another $75 a month per each space. It's easy money and you don't have to be there all day everyday running it. If you built tall big spaces for inside boat storage, you would probably be covered up with folks who don't want to drag their 125k wakeboard boat back home every weekend. There's 3 places by our lake house that always full year round.