1. Raid the girlfriends kitchen ware collection for a glass ice cube:
Or you could purchase one from a cookery shop, florists etc for a couple of quid.
2. Make a mould from two part silicon rubber, I bought mine from TOMPS here in the U.K. they have a very extensive and useful online shop. The cube was hot glued onto the base of a plastic business card box but any watertight container would do. I do stress watertight though, two part silicon rubber will find even the tinniest hole and escape leaving a horrible blue mess.
This will now require 24 hours in a warm place to cure. You should be left with something like this:
3. Mix up an appropriate amount of clear florists resin, this I also bought from TOMPS and pour into the mould. Again leave for 24 hours in a warm place to cure. It is advisable to put a cover over the top to stop dust and small insects getting in, though the latter could be quite cool.
You can also at this stage add any debris into the cube, bones, coins weapons etc, but beware anything too heavy will sink to the bottom very quickly and will look a little odd in the finished product.
Below are some of the finished Cubes fashioned by yours truly.
The top cube has had a small amount of gold glitter added to simulate coins, the glitter elements are actually round which adds to the effect. The lower cube has had a few offcuts of plastic skeleton added.