Our "test tube array formicarium" uses this very handy product, the black jacket got longer to accommodate for the larger insert and rings got merged with the design.
 
You can buy it as a stand-alone solution to provide darkness to a test tube as well as preventing it from rolling and elevating it a little to allow for the use of heating cables under it.
 
You can also buy the "black jacket" and "rings" separately. 
 
Shipping weight: 3.5 grams.