A hole drilled in wood?
I have seen American websites recommend something called "water putty" - don't know what it is, but it seems to be cheap.
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D:How can you have not used water putty!?!??!http://www.waterputty.com/
I've never seen it for sale over here - possibly its asbestos content makes it illegal?
It has asbestos in it?
Apparently, but apparently it is also not on the label (I found an article about lax asbestos regulations in America).
One of the talc mines in America has some natural asbestos in it, resulting in an attempt to legislate against crayons in classrooms...The company has been cleared in the past of asbestos contamination of their product http://www.maacenter.org/news/putty-maker-sued-for-connection-to-asbestos-diseases.html
. What skills/resources do you have available?
. Could you mold and fire a ceramic nozzle?
. Could you machine a metal one?
Someone in florida is casting them with a homemade aluminum furnace.
Depends on how long you expect it to last and what quality of thrust you need. Are you looking for something castable (like aluminum, plaster or water putty) or machinable (drilled out, like aluminum, plaster, water putty, some epoxys...)?
Perhaps a tight roll of paper, like a very thick shell?
Whatever you pick, it's going to take some experimenting to get right.
What fuel are you using?