Believe it or not, you don’t even have to put down a sealer. The way these A/C’s are made they have a foam seal they sit on. Whenever you get it positioned you place a shroud on the inside and tighten up. There are only 4 bolts that holds everything in place. This pulls the unit tightly against the roof and makes the seal by compression.
We tested it with a water hose by spraying around it on the roof and not a drop of water could be seen from inside. I did have a little fun at my friend’s expense though… While he was spraying the roof I watched from the inside. I yelled at him after a few minutes to “STOP! STOP!” Then When I came outside I told him “it’s not leaking.” Had a good laugh making him think we had a big problem.
I agree the biggest job is getting it to the roof but you do need more than one set of hands.