
The depth of the channel is then simple to find.
You need to measure both channel depths and the separation between the interior of the slide. Add the depths to the distance between each side of the slide to guess the width between the channels. Make several measurements along the length of the slide to find the narrowest spot in the slide. Be certain to measure carefully and overestimate the width of frame rails. Then incrementally remove material from the frame's rails as needed to inch the slide onto the frame.
If you are patient you can end up with a good slide to frame fit:

This one has no horizontal or vertical slop but moves forwards and backwards like it is on butter.