Wormbert is a snake consisting of segments, which contain neodymium magnets and are interspersed with carbon-steel balls. The magnets I used, and for which the .stl files are sized, were 10mm x 3mm with 2.5kg of pull each, and the balls were 6mm in diameter.
I'd recommend using ABS or a similar material for the printing, because it seems to stand up better to the slight force required to clip the clips into the segments. However, the Wormbert shown here is made of PLA and has held up well. I always print a few extra clips in case of breakage.
Each segment contains a single magnets, except for the tail and head, which contain two magnets each.
Step 1: Print the Segments, and Insert the Magnets.
The Wormbert in the photos contains 27 segments, plus a head and a tail. I don't know the upper limit in terms of how many segments you could add before the head would no longer be able to support the weight, but 27 works well. I printed extra clips in case of breakage, and I printed an extra segment to show people how it works, because I am a giant nerd. The head and tail need to be printed stood on end, with support, and the resulting scaffolding needs to be removed to make room for the magnets.
To insert the magnets, place one in the clip and then clip it into the segment. The fit is quite snug, to ensure maximum retention, so what I do is use a small pointed object to gently coax the corners of the clip into place. The clips can be used in any direction, meaning it doesn't matter which way the poles face, but obviously the head and the tail need to be attracted to each other, so pay some attention there.
The reason the holes in the tail and the head are so deep, is to fit two magnets at once.
Step 2: Add Carbon-steel Balls.
Simply insert a ball between each piece and the next, seating it in the hole provided.
The joints have a decent amount of play, allowing Wormbert to slither and curl up, but if bent too far will break the contact between the balls and the magnets.
And you're done! Your own Wormbert, here to be fiddled with incessantly. Enjoy!