Just an observation regarding the reluctance to make holes in the roof.
If you drill & tap for the fine metric machine screws available in both brass & stainless steel to fix whatever you're fixing, you can apply some sort of sealing compound or, as I do, you can make & fit a rubber gasket. All you need is a sheet of thin rubber (tyre inner tube?), draw around the object marking the screw holes, cut out & pierce the hole positions with a sharp point like a bradawl & Robert's your Mother's brother.
I've used this method on 2 boats now without any problems.
Note: Taps & their relevant drills are available online from engineering supply companies & are reasonably priced, their web site should tell you the drill size required which is important. I would recommend buying quality 2nd cut taps, you don't really need 1st or 3rd cut taps, just take your time & don't get Biffo with them. Also, buy at least 2 drill bits of the size you need for the tap/s you're using as sod's law will dictate that you will break at least 1 before you've got used to them or finished the job!