What do I use to fill in a 1 inch drilled hole below water line?

My raised spa has a small "spa control panel" installed just at/below the water line. The spa control (4-button) unit goes into a standard plastic conduit (I think it is 3/4" but could be 1/2") that was stubbed out at the water line. The spa controller head is just a friction fit into the conduit with sometype of silicone sealer around the opening. The buttons have now corroded and I want to move the replacement controller to the top of the spa shelf. Meaning drilling a new hole, inserting new conduit, and connecting to origianal conduit somewhere convenient.

I can lower water below this opening (partial drain the spa). What product do I use to fill the hole?
I suppose I'll need to remove (drill out?) as much of the plastic conduit as possible to get a good bond with the mortor.
Should I just pack thin-set mortor into the hole as far as possible, then use pool tile gout to attach a new tile over the hole?

What about just" expoxying" a new tile over the hole and not filling in the hole/conduit - would that work?