It has to fit on the carriage, not the slide so that it doesn't move when you put on more cut. Use Google Images to see a variety. The early Drummond ones were a bit weird with a big hole in the middle to let the slide move through the middle. Then there are the ones that are on a long flat base eg for the Smart and Brown which are designed to fit to the long wings of the saddle. Your best bet is something like the standard Boxford mounting.
However you can be a bit different and swing the topslide round to use as a cross-slide and mount it on the T-slitted cross-slide.