My sister-in-law has a tri-plex that has a three car garage with three
single garage doors. However there are no dividing walls to separate the
garage into separate garages. I need to put up a wall so one of the
garages is only accessible to the renter.

It's about 24' long x 12' wide.

I don't need anything fancy or to cover it with sheet rock.

I've thought of many different ways to do it, but all seem like they're
too complex and expensive.