For a colourful cheap fill, just shred some napkins and mash them up with epoxy, it's possible to fill pretty large voids with a wide range of colours this way. I sometimes use duct tape to help hold the fills in place.
For this kind of project, i tend to use sheets of A4 acetate that i can cut into whatever shape i need, i use a hot glue gun to seal it, make sure you use plenty, even a small leak is a disaster.