Venting helium allows the lift to be controlled and the balloon to be overinflated for a quick ascent and then to vent helium to “hover” at altitude

This design using a polypipe fitting can be built quickly and easily for under £20, but the small diameter makes it impractical at very high alt > 24km. It can be built into the payload and doubles as a foolproof inflator. weight approx 300 grams, but using different plastic pipe or reducing the lenght of the 15mm inflation/vent pipe could get this down.