Here are the rods for the demo function. They are steel, 1/4" x 1/8". I ran keyways down the middle of each, 0.015 deep on both sides. I might add keys to the aluminum blocks that go on the rods so that the blocks don't rotate.
The holes are 0.3125" apart, which corresponds to half the distance between rods (0.625").
I put them into the function block. Some are tight, which probably just means I need to file the acrylic holes a little bigger. Laser cutting acrylic is not very precise.