If you’re gonna install a PVC conduit, I think securing it with an adhesive like 5200 is a far simpler way to go, and it saves the drilling of unnecessary holes in the mast…
I did this about 15 years ago, mine’s held up fine… Simply assemble the conduit (I did one from the bottom, another from the top, to allow a break at the spreaders for spreader, steaming, and foredeck
lights) and run a heavy bead of 5200 or similar along the conduit, then slide it in the mast (keeping the bead from making contact with the mast wall until you have it properly positioned, of course) Then, simply rotate the conduit to where you want it…
Don't forget to run some sort of messenger
line inside the conduit, especially if you're leaving a break somewhere, as I did... You'll be happy you did so when it comes time to run the wires, you can trust me on this (grin)
Then, to insure you’ve got a consistent and adequate distribution of the sealant, slide an equivalent length of heavy, black iron pipe up inside the conduit to weigh it down, and ensure a good, solid connection with the mast wall…
Let it sit until the adhesive cures, remove the black pipe, you’re done…