Small Screw Top Capsules From Plastic Bottle Caps, Necks
Instructables user bfgreen makes small, lightweight, waterproof containers like this by sawing off plastic soda bottle necks right below the lip, flattening the cut edges on a file, applying cyanoacrylate glue, and clamping. Since these bottles are usually PET, they could also probably be solvent-welded with acetone and other common solvent cements.