The model can make quite a bit of difference. For example, on a Ruger Redhawk, the bottom of the spring tunnel behind the trigger near the top is a perfect place to drill out and thread for an overtravel stop, and that's exactly what I did with mine. I doubt that convenient cut exists in your Witness.
In a 1911 it makes no difference where the screw goes on the trigger because the trigger moves straight back and the 1911 trigger stirrups are so thick you can skeletonize them to access the screw from the side, as a Goldcup overtravel screw does.
Your Witness has a thin trigger stirrup, so you can't put much screw length through it without it rubbing your trigger finger. Just looking at a photo on the importer's web site, the place I would be looking is the angled up flat spot at the top of the back of the trigger. It'll be easier to drill into that flat, and you won't have to worry about the screw ever reaching through to your finger. It will also let you use a flat head screw if the only available stopping surface is polymer rather than steel (I've never looked closely at a Witness and can't tell from the photo).