I have seen a picture of an external wave box, not sure where. You would not need the actual wave box, but rather the external box you build would be the wave box.
Possible difficulties could be; drilling the hole into the main tank the right size to hold the stream in place. also you would have to engineer something other that the foam piece that supports the stream since it would have to be install from the top, rather that the bottom sliding apart.
Check out the spare parts on tunze.com, they have all the pieces for the wave box broke down, so you could get a better idea of what you are up against.
Also if you had a 6100, all you would need would be the wave controller and an impeller for a 6200. You could make the controller for about 15 dollars if you have a knowledge of electronics ( I made mine ).
When I build a new tank im going to have an external wavebox. i am even considering making my external wavebox support two streams, might workout might not.:eek1:
good luck.
landy