Speaker Magnets is all I ever use. They come in so many different sizes (weights). You can slide then anywhere along the OTA.
Most Speaker Magnets are only held on to the Speaker with soft aluminium rivets, which file off, or break off, easily. Then I glued bicycle tube rubber to one side so it didn't scratch the paint on the tube, Felt would work too.
Here is a pic of my Magnets, with one tipped upside down so you can see the rubber glued to it.
