I have finally found a supplier who has decent looking extractors for XU engines on the shelf (in Holland).
The actually look pretty good. Individual header pipe for each port, 2 into 1 just off the head then balanced runners about 600mm long to the 4 into 1 collector. The connection at this point is the same arrangement as the original engine pipe - Torroid seal and compression spring connection.
I haven't yet figured out how to post pictures, but if anyone else is interested in a bulk buy, email me and I can send you some further info.