Been meaning to make one of those for the combination chuck I bought last year, even have the aluminum cut and ready. Adding a recess hadn't occurred to me, that's a great idea.
On a side note, I referred to this tool a few times as a spider and got corrected, one time "it's a guard" and another time "it's a stop", and every time "a spider is what goes on the outboard side of the spindle". I dunno, maybe "spider" is like "dog", and gets pressed into service whenever a tool doesn't have an obvious name