the most novel idea ive seen is a piece of tube 6mm diameter whatever fits the nipple, long enough to go into a jar, the open end of the tube is blocked up and a slit is cut up the pipe 10-15mm long with a sharp blade. works surprisingly well, need fairly thick walled tube tho.
But i guess a brake fluid cap, an old tyre or similar valve, suitable sized hose, some adhesive and compressed air (tyre, bike pump etc) should do the job.
Or of course buy a tool...
i started this thread to avoid stuff failing :grinning: