@Charles Moore, I have several garages rented on various properties here in CT. We have a lease just like any other rental, although we've never really had anyone default on a payment yet so i unfortunately can't offer any wisdom on that front. However, if what i know about the storage industry, you can just lock up the stuff until they pay. It's not the person's home so it doesn't fall under those laws. Don't take my word on that though, if the situation ever comes up, i'd call our lawyer first so i don't even trust my own opinion on this. haha
Definitely put up a wall between your space and the rental. and don't use drywall, use at least 5/8 plywood and screw it in. It's not bullet proof, but if the renter wants at your stuff then they have to work for it.
We don't restrict access to our garage rentals either. It's their storage space, they can come as go as they please. We have stipulations and rules depending on the type of rental. If it's just storage then basically they can't do anything and we don't even provide them a power source. If it's a garage that the renter can use for auto work (no businesses) then we provide power (most of our garages like this have separate unities meter and they rent the entire garage).
If, however your two car garage has only one door than this is a significantly different conversation. Your going to want to restrict access when you are there. But this shouldn't be an issue if it is just winter car storage or something like that. It's not like they will need to be in and out a lot. Or, just offer it as winter storage with almost no access during the winter. This way you don't have to worry about it.
I could offer up my garage leases for you to see, but they are all CT based so not sure if they'd do you any good.
-Dave