on my mast there is a loop of thin wire attached to one side of the track and a hook on the other side for an eye in the wire to hook onto, simple but effective. I have seen screw type locking devices for sail slugs but not slides.
Last time I changed from bolt rope to slides I got 2 strips of stainless about 10mm wide and a little longer than the rope entry. I drilled and tapped the mast and screwed the strips over the entry leaving a gap for the slides to pass between them so that I had no problems reefing. I did check that the first slide under tension after reefing was above the entry point for each reef. The only time I had to unscrew the strips was to fit or remove the sail.
Most boats I have seen just have a pin through holes in the moulded track from side to side. this stops slugs coming out when sail is dropped.
It does present difficulties when you reef. Inevitably some slides will need to be removed from the track if you are to get a neast reef with tack down next to the gooseneck. If you want quick reefing from the cockpit this is a problem. Either do without slugs for the reefed area of luff (not good for deep reefs) or you need to oppen the gate.
Stan D may have an idea if the gate is made with variable friction so that a good pull on the tack reefing line will pull the slugs past the gate while normal sail lowering will not allow them to escape. It wouldn't help much when you want to shake out the reef.
I use just a bolt rope which is great for reefing but makes sail furliung more difficult. good luck withthe new mast Did you break the old one? ..olewill
I have a Moody 31 with a Selden (previously Kemp) mast. This has a continuous sail groove with a sprung gate for the sliders to clip in through. The main drops below the gate so that you can slot in a trysail without taken the main off and without needing a double groove. Look on the Selden website and search for 'sail entry gate'.