hmm somebody doesn't understand acoustics..
..oh wait ...its me
ok.. perfect use for them is totally a high freq. diffuser/ absorber !
get a air nail/ stapler gun .. and staple or nail them onto a plywood board
bend the board so you get an arc ( poly cylinder diffuser )
fix that to any wall that you suspect might need diffusion and wont make the artist fall over laughing while they perform
and i am sure it will work well .. better then if the rubber ducky were not attached .. for sure .
.. sand ...?
they are not big enough to have any effect on anything freq. where filling them with sand will make any difference
says by my idiot acoustic brain .
in fact i am pretty sure you could draw diagrams for the duckfuser and patent it and manufacture
and then sit back and watch NOBODY ever buy one off you ever
or steal your design ever !
but there is something about having the only one in the world isn't there ..?
do it .. build it
its nuts enough to have merit ..