We recently equipped our personal auto to tow our newly purchased Airstream, which I am evaluating renting on Outdoorsy. I decided to install a Prodigy RF brake controller rather than one that installs in the auto, both as we don't know how long we will keep our current tow vehicle and to allow us to have the option of renting a different vehicle or to rent the trailer to others without this restriction. The Prodigy RF is a fully proportional brake controller (essentially the same as the very popular and liked Prodigy P3) but instead has a box that mounts to the A-frame of the trailer. That trailer wiring then plugs into this box, the box then plugs into the tow vehicle. If the renter already has a brake controller, you would just plug the trailer wiring directly into their auto and not into the brake controller. There is a small remote that is used to manually apply brakes or to adjust the braking force, it just plugs into a 12V outlet in the towing vehicle.
So far this seems to work perfectly for us. I self installed it in about 10 minutes, just needed a drill and a wrench.