a great use for a used sushi tray is to use it as a mini-germinator! I use a mini soil-block maker (found in many catalogs) with seed starting mix to fill the tray and start my seeds in the little cubes. The clear plastic lid makes it a perfect little green house under grow lights. Once the seeds germinate I can pull out the separate soil blocks and put the plant in a larger pot. This doesn't work for large seeds, but it will work for most. Using cut-to-fit sections from an old thrift store Velux blanket makes perfect capillary matting.
Sushi tray germinator