Interesting idea. However, you could easily make one from a dry bag and one of those spiky rubber ball things you often see in places that sell childrens toys e.g. National Trust shop.
[Edit]
If you used one of the Sea to Summit bags with an Event base, you wouldn't need the valve to vent the air.
Here's some spiky rubber balls that flash. Maybe a bit big at 8cm, but I am sure you can get smaller ones.
http://www.whositfor.co.uk/store/product/34313/Flashing-Spikey-Air-ball/
Edited: 13/04/2012 at 09:44