Hi Paul,
no problem, if you do a little searching, you will find loads of descriptions how to build them... If you want to start with a passive mixer, look here, for example
http://www.nirvis.com/mixers.htm
The upper part only shows a mono mixer (left or right side only), you have to build two of them.
You can of course add potentiometers "behind" the fixed resistors (they used 2k, i think the MB6582 uses 10k, which I would also suggest) for manually reducing each channels signal level.
That should be exactly what the passive Rolls mixer does...
Best regards,
Peter