How should I hang a receptacle on a rim joist?
If I were doing this I would use Carlon Type FS boxes and rigid NM conduit. These boxes can be fitted with water-tight covers and have screws for surface mount to the existing rim joist. I would run conduit under the joist then up to the boxes. Either with a T as you suggest or use one Type FSC box for inline connection. If you need more volume the 1-gang Type FD boxes are 25 cu. in. and 2-gang versions are also available.

If this were a home and not a cabin I might go to the trouble of mounting boxes flush inside the rim joist. But that seems unnecessary here.