With Pete's astute help, I finally built the grills for these things. Pretty easy to build, all in all. Sorry for the bad quality pics.
Dado intersecting pieces to make them square, glued up, then beveled with the table saw.
Taped washers to match up with the magnets inset into the baffle.
That worked OK.
Pete recommended trying them with additional magnets as standoffs to separate the grills from the baffles just a little bit.
I can't tell much difference in the sound with the grills on directly, with the standoffs, or with the grills off.
Not sure which I prefer - baffles directly mated with the grills or with standoffs. Options are good!