Simple Q ... you can get plastic balls (ooh er missus) that attach to shock cord, about 20mm diameter solid items, with a 5mm (or so) hole thru centre.
such as
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don't want to make a tank banger, but I do want to get hold of some of these balls .... anybody know what they are called ? ... and importantly where I can get them.I tried half a dozen outdoor shops today with no luck, but got some pretty coloured shock cord :-)