Bownet's U-Fill sandbags are used for the Gym or Turf fields. Attach them to the steel ring on the bungee.
They come 2 to a packet, with an internal sealable plactic sand bag, complete with flling instructions. The Velcro Strap also doubles as a handy carry handle.
Only 2.2kg ( 5lbs ) when full, if you need a little more weight, (because you have such a powerful shot ) add half a cup of water to the sand