Would be be OK to blast one square hole in the center of the enclosure, instead of (a) waiting for the pre-drilled enclosures or (b) attempting to make the slits?
Yep, if I had to do it over again this is what I would do. This might be the way Tayda ends up having to do it as well.
The new Mesa pedals are all done like this too, FWIW.... One universal enclosure with a big cutout and a faceplate for the controls.
Just make sure you don't drill past the edge of the faceplate, there isn't a lot of room for error there.