The crosslide clamp overview. I made mine from a spare cutoff of BMS. Thus it has a superfluous hole through which you can see the clamp bolt. The thing clamps to the carriage using the 1/4"-26 BSF hole used for the travelling steady. I have drilled an extra hole and tapped it M6, and both holes have M6 Allen bolts in them, though this is not ideal as an M6 thread in a BSF hole is highly prone to overloading the threads and stripping the hole. You have been warned.

Closeup. The brass pad is fitted into a filed slot. The end of the clamp bolts goes into a dimple drilled in the face of the brass pad. All edges have been chamfered with a file.

My alternative design, which I though of as a result of drilling the extra hole the wrong size and having to bush it to be able to redrill and tap M6. This design uses the single tapped hole without any mods to the carriage.

The clamp pivot piece [1] itself is allowed to rotate around the hold down bolt [4], and 2 brass pads [2] (retaining them left to the maker as an exersize) bear on the cross-slide. The horizontal bolt [3] performs the clamping action.

PS: the hold down bolt [4] is 1/4"-26 BSF (British Standard Fine - a Whitworth form thread)