When I was looking for a nozzle rack, I did find standalone racks, and racks that attached to other printers, but none that attached to the Anycubic i3 Mega. Since I have very limited space around my printer, it is convenient for me to have things attached to it... so here's a nozzle rack.
The one that I printed is the one that goes on the left side, and has numbered labels to show which nozzle goes where. The numbers are basically the nozzles that I have, and a couple that I intend to get. If you have different nozzles, you can optionally print the version with no numbers and find your own way to label it.
For the version of this rack that goes on the right hand side, I'm pretty sure it should work fine, but I didn't actually print it. To make it I mirrored the clip (which should be enough) and for the rack I mirrored the threads and the numbers (on the numbered version) and then mirrored the whole part bringing the threads and numbers back to the correct direction. I think that should do it.
I have designed this so that the clip stays on the printer all the time, but the rack can be slid out from the clip and removed. I thought that would be convenient.
The nozzles themselves actually thread in to the rack - I did this because I didn't want the chance of dropping the rack when I detached it from the clip, and dropping nozzles all over the place. The threads are M6 which works with the stock nozzles that fit this printer and most nozzles that I have seen.
The tolerance on the threads is a little bit tight, but I found it was workable. Even though they were all designed the same, with my print I found that the holes on the outside edges threaded great, but they got progressively tighter as you approached the center hole. I did find that on the tighter ones they loosened up okay just by threading a nozzle in and out several times. I've also found that a bit of WD-40 often helps with this sort of thing. Lube it, thread it in and out a few times, then leave it threaded in overnight. Depending on your machine calibration, it might even just work without all that... or you might need to get more serious and use a tap. In my case I decided it was only really important for it to thread easily a short distance anyhow, since threading them all the way down just makes them more difficult and time consuming to remove, and the only real purpose behind threading them is to lock them in place so they don't get dropped.
Final thoughts: Probably a good idea to make sure your nozzles are fully cooled before trying to thread them in to the rack.
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