I designed some test tube plugs with nest entrances. I'm planning on making them with three different size entrances. In addition to plugging the test tube, they also keep the tubes from rolling around. These are made of silicone, so they grip most surfaces and won't slide around.
I'm already in the process of making a large mold for these, but if anyone has any ideas for any design changes let me know. Maybe there's something I didn't think of.