Exactly Bob...that is exactly it.
There is another fellow on ebay who makes a brass tube lined wooden block piston that is much less expensive and also quite efficient. A friend of mine has an article on it, but the magazine site is down right now.
The Aune pistons are works of art. I got mine for around $40 on ebay and it is worth at least twice that. My Wagner piston cost quite a bit more, but it too is awfully nice. The top of it has a tinder cavity.
Darrel Aune has a DVD on how to make a piston...just watching it convinced me that buying one was the cheap way to go.