If you're after a home remedy then here's a couple of options that I would suggest.
A- use some plug screws from below so you don't see the slot and then file off the ends that stick out down flush with the slide's surface. Some tape or pop can side metal can act as a shield to aid in avoiding damage to the slide. Then remove the screws, blue or Park the ends as required to match the slide and re-insert them with some red or yellow Loctite.
B- Similar to the above but this will be more permanent and look a little better. Insert the same sort of plug screws from below. Cut them off and file until slightly proud. Like about 0.15 to .020 high. Make up a hard rod or grade 8 bolt "anvil" that you can securely clamp i a bench vise and shape the end of this anvil so it fits up into the slide and rests against the end of the plug scrrews. From the top use multiple LIGHT taps of a mid weight ball peen hammer to mush down and spread the screw. The technique is the same as you would do for a riveting operation. THis will spread out and lock the metal of the screw to the slide. It also swages the metal to fully fill the threading. Once mashed down nicely dress off the slight humps of metal, polish and re-blue or re-park or whatever sort of finish is on the slide.
The threading won't go away completely with option B but it'll be FAR less visible than option A. It'll basically look like a couple of small wear or impact marks instead of an obvious threaded hole.
The nice thing is that doing it this way avoids the risks of welding that may upset any heat treatment in the slide metal or result in warping due to the shrinkage stress of welding up the holes.
Stuart
