Wayne's M/S board is great. And at a really nice price as well. And, because of the jumperblocks between the circuits, very versatile.
The basic version is like this :
Debalancing > MS encoder > balancing >MS insert> debalancing > MS decoder > balancing.
I've got mine setup like this :
Debalancing > MS encoder> high pass filters > balancing > MS points Patchpanel> debalancing > gaintrim> MS decoder > balancing. All switched with relays by the way. With a true bypass for the whole. I'm having the parts for a second unit ready, I just need to start soldering. They'll be sitting in the first 2 stereo subgroups of my console.
I know, this is not what you're asking for, but it's worth to have a look :
http://www.ka-electronics.com/KAelectronics/MS_Matrix/MS_Matrix.htm