Part of the innertube with the valve holder.
Cutting away the excess rubber.
This is how to assemble.
Full assembly.
The valve holder goes into the Gardena coupling at the left. At the right it pops out and one can assemble the whole valve.
The result.
In my launch video’s you can see I put the Gardena on top of a small base plate with two long spikes that is easy to push into the soft Dutch soil. A spike sticking up sort of holds the rocket upwards while pressurizing. To depressurize the system I just need to unscrew the valve nut. The most right nut at the right in the picture directly above. More info on Dutch bicycle valves can be found here.