Making system sized bricks is quite a challenge because they have thin spots. Duplo or quarto or would be a bit easier.
It will take a little skill and patience but making your own legos is worth it.
This method requires no special tools(but if I had a mill I would mill them out)
Step 1: Plot It Out
I measured the bricks with a caliper and got some good measurements. I provided them in millimeters, inches and fractional inches (to the nearest 1⁄16 so it may not clip to plastic blocks)
The right view is shown only for completeness (it is not needed for dimensioning)
Step 2: Drilling
but it is where the studs of other bricks clip in. This means the placement is very critical.
a 3⁄16 drill bit is the bit I used.
I strongly suggest leaving space around the actual brick so it does not fracture.(trims nice later)
Making an indent with the drill bit or an awl, or center punch will make it a little easier.
(I cheated and used a thin piece of plastic as a template so I only had to plot once)
Step 3: The Studs
I cut them about 1⁄4 (7mm) length, just eyeballing it. Then chamfer at least one end (makes it easier to put in)
It is best to run the dowel through a 3⁄16 hole in a piece of metal or plastic to size it.
The dowel was a little larger than 3⁄16 which causes problems putting them in.
I used a punch and hammer to gently nudge the studs in their holes.
If they are angled try to straiten them by tapping the higher side.