Foam ... hmms, haven't - yet .... sounds quite feasible though. Could cut it small enough if one wants to dub over it I suppose.

I have been guilty of adding hollow-hair (ie/ polar bear) to my size 16 and 18 parachutes for added floatation support. I lay these along the base wrap and head on from there (covers them up). As I tye them to sit in the film, they tend to take on water easily enough - and .... PB hair helps keep them there. I can see myself trying foam next .....