When pre cutting a lot of cross members for a layout build there was always a lot of saw dust mess from dozens 1 1/8" holes.
By reworking a 90 degree 4" lawn drain elbow I could slip it around a spade bit, turn on my Delta super sucker, partially visible in background, and have a dust free zone. The lawn drain elbows are available in a tight 90 which works well with standard bit length. Note how one female end of the 90 is removed to get the fitting right onto the work surface. Some stores have rounded medium sweep 90s which would not work as well. Compare the two 90s. one cut, one uncut.
You could also use a 4" to 2" reducer to use a typical shop vac.
Using the mid 1/3 guide line I would use a 1 1/8" bit for 3 1/2" material. Drill half way through one side then flip over to bore out the balance from the other side, That will provide a clean edge for finishing.
Here is a stack of cross members for a module. I use a 1/4" round over bit on all openings.
Cut a bunch more holes than you think you will need. Easy now, pain later.
Note: waffle bottom skins and a taped off floor in anticipation of benchwork encroachment.
