Approximately 1.5 inch /30 mm dia steel balls are perfect to weigh down the spool in the AMS-I bought them from Amazon.

7Pcs 1.5''inch Bearing Balls Steel Ball Bearing 38mm Assorted Loose Bicycle Bearing Balls 1-1/2"…: Amazon.com: Industrial & Scientific

However hard to keep them without rolling all over the place when not in use. I designed this skadis board mountable dispenser that you can use either freestanding or mounted on a skadis board.