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.
Commenters on bfgreen’s post have responded with their own versions of the same idea. This small case for holding airgun pellets, above, is by Instructables user CaseyCase. Below, a micro spud-gun ignition chamber from bloke2022.
Update: Contrary to what I suggested above, experiment has proven that acetone does not effectively solvent weld PET in this application. I haven’t tried DCM or DCE yet. Heat-welding the joint does work, but its effectiveness compared to super glue is questionable.