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)