There are three commonly used type of stopcock plugs for 'simple setups'.
1. A glass stopcock plug (left)which is held in place by a spring and small metal clip. Both parts are needed to hold the plug in place.
2. A glass stopcock plug (middle) which is secured with rubber and a metal retainer clip
3. A teflon (glass) stopcock plug (left) equipped with a thread and nut.
The glass stopcocks have toi be lightly lubricated to get a better seal. It is advisable to check it the hole of the plug and the stopcock match up and if the stopcock plug fits tight. Plugs made from glass have different slopes that Teflon plugs and therefore are not interchangable.
Please note that Teflon plugs shrink significantly when placed in a cold environment (i.e. freezer), which causes them to leak!