I can tell you where you can get the felt covered fuel clunks. I am a heli pilot mainly and on one of Robbe-Schluter's machines called the Futura SE it come with one of those clunks. It is used to stop the clunk from picking up air due to all the extreme aerobatics helis due and the clunk being thrown around. The part number is S4154. You wil have to get it from a shop that sells Robbe machines. There are several on the net. Ricks R/C.
www.Ronlund.com
I use them and love them.
Norman Ross