You could use spam, but I think it's a bit too opaque.
Very thinly sliced, of course, and a degree of transperancy does become apparent. Not too sure that any of the big players will start using it soon, problems with increasing the magnification beyond the power of 1 for starters, and 'issues' not so much of waterproofing, but of full degradation of coatings AND lenses in damp conditions (plus a funny smell of course).
There are probably a few other issues, but they would be quite cheap.