I see some improvements I think might be worthy of playing with. The first and most obvious is to make the shell out of lighter material....silk or cuben ala Bill Fornshell. The second is to make the shell 'baffled' like a quilt, with the baffles fitting tightly around the balloons. This would reduce the gaps where drafts sneak in, and may improve the R-Value.
The thrid possibility is to tie the balloons so they CAN be re-used. There is a technique involving twisting of the valve instead of tieing a knot that would work, but the balloons would ideally be more robust for this approach!
Definitely a fun DIY project ;)