Last year, I sold off 27 cymbals. I used boxes either 1 inch or 2 inches larger than the cymbal. So, for example ..... an 18 inch cymbal would go in a 19x19 or 20x20 box. Typically, boxes are gonna be taller/deeper than they need to be. So I'd make the box (tape up the flaps), and then cut down the height. (Cut out the middle of the sides. Don't mess with the integrity of the ends) That leaves you with 2 cardboard trays that will be your top and bottom lids. About 3 inches of box wall height is what I'd leave.
Then I'd wrap the cymbal completely in bubble wrap, and place it into the bottom tray I just made. Take up any extra empty space with more packing material. Put the top lid on (you'll have to pinch the bottom corners a bit) and tape it up good and tight. Cymbals shipped from California to as far away as Florida and New York, and all arrived safe. No problems, no complaints. All shipped thru UPS.