Does it need to be a Kensington slot, or just lockable? There are some cases that have a tab with a hole that slots through the side (or top) panel when it's attached. Running a small padlock through it blocks the panel from coming off and lets you attach it to a length of cable or chain.
Not sure how you'd find those, either, beyond word of mouth, as they seem to get photoshopped out (my CM 690 has one IRL, but not in product shots) and don't show up on spec sheets, but it might broaden your options a little.