These plugs come standard with all of our formicariums, made by combining vinyl tubing and a plastic piece on one end. The vinyl stays flexible, and the hard part grows the vinyl diameter slightly, creating a perfect seal against the opening.
Best of all, they are reusable; if the vinyl tube wears off, you can cut a new piece and re-use the hard plug portion.
The plastic piece is made with ABS-like resin using UV resin 3D printer.