Paul,
I just wrapped up 5 years and 35,000 miles towing my 2000 5-speed behind our RV.
I bought a new PRHT.
http://www.ntahc.org/2007/0002.JPG
I have helped over a dozen other Miata owners to get set up for towing.
The Roadmaster set-up is $400+. I have a source for a NA-NB baseplate for $100.
You can probably be up and towing for $250 or less.
Contact me at tim@ntahc.org if interested.
Tim