Wow, here’s a really creative way of making your own LED cubes!
The material used is Smooth-On brand “Clear Flex 95″. This is mainly sold as an industrial product, although smaller quantities (highly recommended! See below for why) may be availble from art supply houses. It comes in 2 separate jugs of syrupy liquid marked PART A and PART B, which are mixed together in a specific ratio by weight.