Snake charmer target question

[TRPrecision]

I am trying to build a snake charmer target and can't seem to get the buoyancy to weight balance anywhere near correct. Has anybody built one that can give me a suggestion of what to use for the float. Just can't seem to find a good option for what to use and where to source it.
Thanks

 

[6.5SH]

Look up the Yuma match masters on Practiscore under NRL22, likely will be able to contact the right person Vs the main page.

Yuma MatchMasters - National Rifle League 22

They have one and also provided the guide to the NRL on building a tank trap.