Take a cardboard box and cut it at the seams into squares a little larger than your cymbal. If you have a 20" cymbal, the cut piece should be a square 1-2 inches larger than the cymbal.
Place the cymbal on the cut piece, use a pencil and mark out a 20" circle around the piece of cardboard, use the edge of the cymbal as a template. Cut another piece of cardboard a little larger. Cut the corners of your squares round, leaving and inch or two over, you can also do this in the folding process for a better fit till you get it down.
SARAN WRAP, plastic wrap, shrink wrap... a box is like $2.00. Wrap your cymbal with plastic wrap until you feel it has some protection, maybe a 1/4 of the box, depending on the mil thickness of the wrap material. Stretch the wrap when you go around the cymbal in cross patterns covering all the exposed metal completely/throughly, less is not more here. You're shrink wrapping the cymbal.
Lay the wrapped cymbal flat on the smaller piece of cardboard, then put the other larger piece on top sandwiching the cymbal in between. This next part takes some wrist/finger strength- fold both edges of the cardboard around/underneath the edge of the cymbal. Use packing tape to secure the fold, then start working your way around the cymbal. Like the plastic wrap, stretch the packing tape like you would electrical (friction) tape for a tighter hold. Your subsequent 'box-sock' is going to be covered in tape, just like the cymbal is covered in plastic wrap.
If done correctly, this method yields the smallest box dimensions possible and provides excellent protection.
A giant cardboard frisbee with a cymbal inside.
By comparison, putting a 20" cymbal in a 20x20x20 square box yields a box that too big, also there's no way to secure the cymbal inside w/o piling up a ton of shipping charges in weight.
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