This is exactly what we use for waterproofing our fireworks before a display. Its clear so you can still see through it, we like to also use a fluorescent post it note where the fuse is so we can see it clearly. These are extended core rolls meaning very easy to hold while wrapping, something again we find much easier.

The price is for a single roll which is 50cm wide and I belive 250m long. Its pretty thick too again I think 25 or 33 microns. 

We wrap our cakes in both directions meaning a double layer of protection. The fireworks will easily burst through the plastic but if you get worried simply remove before firing.