Another way is to use a rubber block(cylindrical) about 2mm smaller than the inner bore, then drill through the centre, put a large(almost same size as rubber) metal plate either side, bolt through the lot, when tightened if using a soft rubber it will expand outwards sealing the end.
The end with the gauge simply use a bolt of the fine pitch as used in a std valve, drill hole down centre of bolt, this can be clamped like normal but lets you put air in at the same time.
tabetha