I designed this to fit a Swanson hexagonal plumb bob. The inside edge of the opening has a small lip, so when the plumb bob goes in, it goes in snug. Once it's past the lip though, it's looser (though it doesn't rattle) and is easy to pull back out. The end of this bob has a metal ring used for hanging the plumb bob. The end of this spool has a couple of pegs for that ring to attach to, to keep the spool from unspooling.

The magnet channel can hold five small magnets. It was designed for these: 

The magnet dimensions are: 2mm tall with 6mm in diameter

Even the strongest magnet of that size won't allow the plumb bob to just hang from the spool when it's attached to something magnetically, because the bob itself is quite heavy. The magnets have other uses, but you don't have to add them.

Timelapse: https://youtu.be/ICP228KTF9k