I built one for a buddy a few years ago for drilling shelf peg holes in cabinet carcasses-
Just an aluminum housing I milled up,drilled and reamed for bushings and ball thrust bearings.I used stock gears that yielded the 1" center distance.Each other spindle runs reverse of the one next to it,so for 15 spindles there are 8RH and 7LH bits being used.
You wouldn't have to run gears,it could be cog belts or even roller chains if the system is designed right.
The head I build used fixed center distances,adjustable centers can be done,it just requires more parts.
I just need one more tool,just one!