I designed this 1-2-3 block with ½" holes in FreeCAD. There's a 1mm chamfer around all faces. Print in your material of choice, so far I've only printed it in PLA. Walls and infill are entirely up to you and how strong you want it to be. I printed it with its largest surface on the bed. I don't recommend supports. I printed the first one with tree supports and it took quite a while to dig them out. Second one bridged fine for me without supports, but you may need to tune your slice to get good bridging at the top of those holes. Sanding or drilling can clean up any saggy layers.

Photos are from after sanding and filing to smooth the surfaces and holes.