SELDEN has just the hardware you need - SS slot fittings cost less than $20 ea & sheave boxes cost about twice that. They come in various sizes.
I recently added two Selden slot fittings into our aluminum mast. One rivit secures the fitting in place.
Remember... Measure Twice - Cut Once
First I masked the area with blue tape, next I made a paper template to be sure of the cut I was about to make, then I traced the template onto the tape, next I drilled pilot holes in center of end radius, then enlarged holes to to proper diameter and finally carefully cut material between holes with metal blade in a jigsaw.
That was the easy part.
Our mast is 66 ft and now has six internal lines for halyards & topping lifts. Some scoundrel ran off with all of the lines while the mast was stored in the boatyard - before we bought her.
With the mast on stands, we used a 100 ft wire snake to pull each halyard (one at a time) and cranked hard on the winches to make them "bar tight" in order to remove sagging and get them off the inside wall so as not to interfere with the next runs. When the three stbd halyards looked good we rolled the mast 180 degrees and repeated the process. Luckily, there were conduits for all the wiring
and none of the internal lines got twisted around anything in the process.
If the mast is stepped... first tighten all other halyards to make them plumb within the mast. Then list the vessel about ten degrees to the intended side which new halyard is to exit mast. Go aloft and feed a short length of bicycle chain attached to a messenger line over the sheave and employ gravity to find the most direct route
to the new slot. Have someone on deck
looking for the chain to appear at the slot and use a bent wire to fish
it out through the new slot. It's better to run the new halyard outside the mast up to the person aloft and feed it down while the person on deck
carefully pulls the messenger. And it's much easier to accomplish if you wait to install the exit hardware last.
That's how we did it, anyway, and I'm sure other people will offer other ideas.