I've just been through the same thng (with a Libretto). I built something out of MDF, and cut out the front to the same size as the Libretto base. Then, I used L-shaped brackets screwed into the MDF base, with felt pads on them to hold the Libretto in place. There's one bracket on each site, and the felt padding really stops the Libretto from jumping out.

However, what I haven't done yet is thought up a way to make the whoel mount easily removable. My car has a free DIN slot under the radio - what I did in my MPV was take out the cubby-hole box that was in there, screw the Libretto 'shelf' to it, and then screw the cubby-hole back back into place.

I think this has got potential, and although it's not as nice as some of the mounts around, it's a lot cheaper.