TY for the detailed pics, it shows the need for finesse / creativity
especially as that line /dado /groove is only at the top of the panel
could some delicate work with a table saw / router allow a blades/ bits "thickness" trim on the left/right/ bottom edges to create that spacing or the look of it / a minor dado on all 4 edges?
if so, then that's a step toward uniformity.
while it sounds like a PITA, it will provide either an accent or transition "line" between the cabinet edges and that front showpiece panel.
next is to find a thin flat headed security or elevator or carriage bolt to be recessed flush into the front of the panel. i.e. not using the supplied bolts.
a recessed nut on the backside will prevent any spinout issues.
it is a permanent install.
you use the same internal wingnut /washer attachment setup
so it can now be uniformly / completely veneered / formica'd .
personally, not having the bolts/capheads showing > a better look ?
why am I thinking of the classic Frankenstein "bolts showing in your neck" meme while composing this ?
that look isn't for just anybody . . . or any sub I'd want to occasionally look at without cracking up- JS
now, about that "fitting in with the decor" issue . . .
what should / must that look like?
presumably, you're not putting these subs in "The Laboratory" for tunes while you're waiting for a lightning storm . .