Revised slide for Octos stable microscope mount

I love Octos [stable usb microscope stand](https://www.thingiverse.com/thing:2525155) but the slide knob brackets were a bit weak due to the way that I needed to print this part.

I split up the part into three different pieces and added a key so that you can fit and glue the parts together after printing, making for a stronger slide.

If using PLA, you can use CA or superglue for this.