To a great extent the answer depends on the size and design of the mast and the position of the slot.
I am assuming that you are talking about slugs going into a bolt rope type track. Where the feeder slot is close to the boom and short in length, the simpliest solution is to either put a
stove bolt or machine screw through the center of the track just above the feeder slot that blocks the bottom of the track. If you take the mainsail off of the boat very often a gate will work in this situation. A gate is typically a piece of flat aluminum that has a bolt through one end and a slot in the other. The flat plate rotates across the slot on the machine screw through the end and the slot in the other end of the gate drops over another machine screw on the other side of the slot closing the slot.
On bigger boats where there is more length between the slot and the boom. The more common practice is to have slot covers. Slot covers are angle shaped pieces of aluminum that are shaped to fill the feeder hole and when bolted into place allow the slides to pass the slot. I have had to make them for my past two boats and they are not all that hard to fabricate. They do take a fair amount of care and precision because the bottom of the closer plates have to
line up with the bottom of the sides of the luff groove. Depending on the shape of the mast section the plates may need to be bent in order to lay flat on the mast.
Jeff
