@atomiktoaster - I think you could use a 1-9/16" Forstner bit to make these. If my calculations are correct, this makes a 39.69mm circle cutout.
https://www.amazon.com/Roman-Carbid...=2025&creative=165953&creativeASIN=B00CCBSFWU
The bottom portion can be a thin sheet of wood (1/16" thick?) glued to the block after the circles have been cut. The slots can be made using a smaller Forstner bit and a bandsaw for straight cuts. I suggested 1/16" because 1/8" would be too thick.
You could cut the circles in the bottom piece and glue it to the block, and then cut the openings in the block and straight sections in the bottom at the same time. Sand these areas smooth.