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This is for the Ender 3 PRO, it will NOT fit the "normal" Ender 3!!

Attention: some users still have interference with the bed. On my printer there is still 1-2mm of travel left, maybe the y endstop is a few mm further back on my printer.
Please check the clearances on your machine before starting a print!

UPDATE 02/22/2020:
Wohoo! 1000 Collects and 700 likes!
Thank you all! If you have an idea for an Ender 3 (Pro) thing: just leave it in the comments!

UPDATE 01/19/2020:
Updated to v3
Changes:

• moved optional embossed Creality dragon logo to the top
• tighter fit
• slimmer design, lowered the height by ~2mm
• removed initial version because of possible interference with the bed.

Advise: you can print it in the given position even without support - it will cause slight spaghetti at the clip, but that doesn't really matter, it fits tightly nonetheless. But you can use supports of course (only for horizontal parts, so that you only need supports in the clip itself, the rest should print fine. Or you can use tree supports)
You can also print it with the top facing down, no supports needed then.

UPDATE 03/18/2019:
I have added v2 of the cover here too, because one user mentioned that the original cover blocks the bed from moving all the way. I rechecked it with my printer and it blocks it by maybe 2mm, so it may only interfere when you use ALL of your bed surface to the last mm (which is always a bit risky)

There is a new version out which is a bit shorter and should not interfere with the bed at all, has nicer edges and the logo on the front, not on the top
https://www.thingiverse.com/thing:3490301 but you'll also find it here.

I tried some of the covers here on thingiverse - those that slot into the v-slots. For them I had to disassemble the belt tensioner and both of them broke during assembly. So I deciced to create my own cover and make it a simple clip-on that you just have to print and snap on the white axle without disassembling the belt assembly. And of course I wanted the cover to look good and match the angular style of the printer.

There is one version with the Ender Logo (quite difficult to print in this small dimension) and a blank version included.

I strongly recommend printing it with 100% infill to get maximum strength, especially for the snap-on clamps

My other designs for the Ender 3 (Pro):
Octoprint Raspberry Touchscreen Box Remix https://www.thingiverse.com/thing:4062202
Flexible Filament Extruder Remix https://www.thingiverse.com/thing:4005786
40mm Fan Duct https://www.thingiverse.com/thing:3949647
Magnetic Tool Holder https://www.thingiverse.com/thing:3508230
Extruder Knob https://www.thingiverse.com/thing:3475324
Spool Inverter https://www.thingiverse.com/thing:4196391