Peter,
I got to use the CMS at the Festool training center, I really want one but, like the OP, being in NA we can't get one.
Thinking this through a little more----
Clamp your guide rail to the MFT to, this will give you a vertical stop.
Use the Rapid Clamp to hold the box to the edge of the MFT with the top side (of the box) butted to the bottom of the guide rail.
Loosen the guide rail, set it to your cut line, re-clamp.
Make first cut, loosen Rapid Clamp, rotate box, butting to guide rail and MFT. Do not at any time loosen the guide rail. Repeat for the other sides.
I strongly recommend a box with a soft filler on the floor below the box, to catch the lid when it falls.
Scot,
They are great clamps, some photos of them holding the guide rail vertically, I cut the opening wide so I could do this demo. Need the photos for another forum, to show it could be done.
This is the install that created 44 liters of dust. If you take a close look at the outlet area, you can see the radius corners from routing the openings.
Tom