I wanted to be able to set an EcoFlow River 3 Plus in the bottom 4u of StreelyDan's printed mini rack, but after assembling the original I discovered the battery didn't quite fit. 

This modified side rail is my solution. It mounts to the vertical metal rails with only one screw (M4x30 BHCS) on each side instead of two, but seems to still be very sturdy once the cross rails are attached. This is a remix of the extended length side rail (10" front-to-back size). 

I added 4 extra pairs of screw holes for mounting up to 4x additional cross rail pieces to form a “shelf” for the battery to sit on (requires +16x M4x16 BCHS). A bit of double-sided acrylic tape on the cross rails prevents the battery from sliding around inside the rack. 

I printed mine in ABS with 40% infill, 4 perimeters, and 5 top and bottom layers.