JL
I don't have a problem with oil based - those cans last for far too long. Besides, I really don't want to have oil based paint in the playhouse.

The 1 gallon cans of latex rust, skin over and what not - so I would prefer not to store partial cans if I can help it.

As I see it, I have several choices.
Buy metal quart cans and store the leftover paint in them.
Buy plastic containers (screw top or whatever) and store paint in them.
Cut out disks of plastic or pieces of plastic wrap and lay them over the paint in the 1 gallon (or 5 gallon) cans.

Any other recommended alternatives? We will probably never completely use up an entire can. We will always have partial cans.