Hi Orchidius (dear Bxx),
mounting a correctly sized plywood
pannel to the steel hull
(cleaned, de-greased) with Sikaflex or similar polyurethane
based compound is easy to do.
But.....bear in mind that the stretch of wire running from the ATU to the base of the wire antenna
outside IS part of the antenna
and usually a high-voltage part of the antenna. I really advise against having that situation. You should try to locate the ATU very close to where you go through the hull
to connect to the antenna wire. Once again, if you want to, I'll come over to have a look.
In my boat, to be able to install the ATU just at the through-deck isolated connector the the antenna wire, I had to glue (with sikaflex) a plywood
pannel against the top of the port aft locker so in fact the ATU is "hanging" from the top of that locker and has been there sonce a few years. No issue with this way of fixing a plywood pannel.