After reading some more about ultrasonic conduction (including some comments on this project page and on the Hackaday blog post on this project), I'm concerned that not very much vibrational energy will actually get to the heating element tip. Here's why:

Notice how much space is between the top of the transducer and the bottom of the threaded adapter. I think for this to work really well, I'm going to need to change two things:

1) The threaded adapter should be flush against the top of the transducer to get good mechanical conduction

2) The threaded adapter should be made of metal. This will present a much closer mechanical load impedance to the transducer. Some people were worried that the heating element might melt the adapter, but I actually don't think it will. The heating element doens't get too hot that far up, and the original construction had the heating element directly connected to plastic too. I'm hoping that making that part out of metal won't screw up my thermal conduction and keep the tip from getting hot enough as it conducts heat away into the large thermal mass of the transducer.

So now the question is, how do I make that part? If I had a real shop, I would just lathe it out of a single piece of cylinder stock and then use a die to thread it, but I don't have a lathe. So I can either get one (mini one from Harbor freight maybe?) or pay somone to machine it, or 3D print it. The cheapest place to 3D print it would be Shapeways, which would be $51 in 12 days. I think I'll get someone to machine it for me, since it's a stupid easy part to make. Will probably be cheapest that way. I would do it myself in TechShop [I miss machining stuff :( ] but I can't pay $150 a month for that.