How do I make them? There's nothing about it in the wiki, as far as I can tell.
Slip emitters are one of the scripts actually in the main campaign. What I did is I identified the scenario that contained them then copied it into a new project folder and renamed it save.cw3. Then I got the script when I opened it in the editor and followed the method for extracting images to get the image.
Once you have the script and the image, the rest is as simple as any other special unit.
Which mission is it? I looked at the missions and couldn't find the slip emitters. Not the slip emitters that are unstable, the slip emitters that stay in one spot and can't be destroyed unless you destroy the main emitter.
They're on Chanson (bertha's) and the asteroid map with the bomber tech. Probably on others too, but I can't remember any other story maps that have them.