Thank you for the comments, which put me on the right track. My screws were roughly pointed, so I improved the finish on the point. The original gibs weren't entirely scrap metal, but the indentations for the screws were certainly in that category, so I made wider, deeper indentations, with conical bottoms. Wanting the tightening of the screws to push the strip up into the dovetail, I made the cone angle of the indentations a bit wider than that of the screws, and located them carefully. The result was quite pleasing:

The strip seems to be making full contact with all the dovetail (as much as remains after the manufacturer's rather enthusiastic corner relief) while the other faces of the strip don't interfere with the slide-saddle contact (that's a blob of oil at the top). It's more secure than it looks, and withstands vertical forces a lot better than before. Thanks, all.
Mark