Wanted for a Cox Engine

Metal or plastic

Sir,
Regular rubber bung fits these...
The 30cc one is a moisturiser bottle pinched from a hotel bathroom.
The 90cc one is - you may know already  
Regards

P.S.
Long ago used 2cc plastic syringe for short run on Cox 049 powered free flights

Brilliant!

Any suggestions for a small clunk?

In a 30-50cc tank the pickup tube will probably be too short and stiff to let the clunk move.
You could try using cycle valve tube with a 1/2" piece of copper tube as a clunk (from old unused control line tank), though I forget wether it can survive glow/nitro fuel.
If all fails avoid sustained inverted flight