These .dxf files were made with LibreCAD 2.2.0-rc1. Here were my steps to build a concrete Let's Split case (the steps build a case for one half):

1. Buy a pre-primed dimensioned 1x6 (0.75"x5.5").
2. Cut out the parts in cuts.dxf, plus two 12" sections.
3. Assemble the parts into a mold according to assembly.dxf. I used star-drive 1 1/4" screws to sandwich the parts between the 12" sections. I used the measurements to the x axis to pre-drill the screw holes.
4. Spray a couple more coats of gloss primer into the assembled molds. I'm not sure if this is necessary, but I did it.
5. Print the template in template.dxf at 1:1 scale. Compare the printed template to your Let's Split PCB and plate holes to ensure it is to scale. Use it to place your standoffs -- I used silicone rubber plugs in the molds which, when molded, made wells that I epoxied standoffs into.
6. Use silicone bathtub caulk to fill/round out all of the edges inside the mold. I used my finger to apply it, plus denatured alcohol for cleanup.
7. Pour concrete and finish according to typical concrete countertop instructions. I used Tuff Duck brand countertop sealant from Amazon, applied according to the manufacturer's directions.