Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:2058190.

Covers the opening at the bottom of each tower on a Kossel / Delta printer. It has a rectangular opening for the timing belt, with some clearance to allow for slight variations in position of the pinion. It's designed to be easily removable if you need access, with 6mm teeth to slot into the extrusions. I made this for the Anycubic Kossel Linear, but it should suit other Kossel printers with 20mm extrusions.

The idea came to me after I dropped a couple of screws and an SD card down the hole...

Update: I've added another version with a cable slot, which makes it possible to feed the extruder stepper cable into the channel on one of the towers. The SketchUp file is also updated and includes both models.