The suggestion was brought up-- in the Craftsmen
discussion here-- about using a slot cut out rather than
individual boring with a hole saw.

I think there's merit in this after I've seen how small
variations in vertical line arrays can be visually apparent.

If the goal is to get the drivers as close together as
possible, the slot system has merit.

The mounting surface could be made in a frame of 4
sides and I think that at least dimensioning could be more
accurate (eg. driver widths not matching hole saw cutouts
exactly and thus making the screw mountings problematic.

Only drawback that occurs to me right now is the shape
of the face frame of the driver. This would ideally be as
square as possible.

Are there any further thoughts on this?