That single rod in the middle is 1/4" in size.
Use a thread-cutter on it.
Of cause you can change it into a hole for a 1/4" or 7/8" bolt or change it's size to 7/8" since it's parametric.

The 3 bearing-holders are attached using M5 sinkhole bolts.
Their holes have 0.2mm instead of 0.1mm allowance to let them turn more freely.

Slightly bent rods or rods being not precisely 75mm apart should be counteracted automatically by the 3 bearing holders turning.

The slider has a clamp-mechanism, so you can later upgrade this design to a motorized slider.
The top 1/4" holes in the ends then each hold an axis for a belt/rope.

Print the "Slider Center Base" at the lowest possible temperature as it's an invitation to warping.

printed parts:
2x slider end of the parent thing.
3x bearing holder
1x center plate (already prepared for a later upgrade to a motorized slider)
non-printed parts:
2x 5mm precise rod that doesn't bend
3x LM8UU bearing
3x M5x15 or longer sinkhole bolt
3x M5 nut