Tim...please speak up if you have customer experiences in this type of endeavor... c'est vous plait!
My 6700 and accessories are mounted in a Gator 6U shallow case:
Everything except the 6700 is secured to shelves with small strips of Industrial Strength Velcro (cut from a 15’x2” roll). I don’t believe Flex has any requirements for bottom clearance for the 6000 series. If that is the case you could remove the 6300’s feet and apply a few strips of the IS Velcro to the case bottom, which would easily keep the Flex from sliding around on the shelf (assuming you don’t throw it into the trunk upside down, etc.). Removing the feet would also give you a little more top clearance.
The IS Velcro is quite strong and a little goes a long ways. When I first started using it I overdid it and found it was difficult to pry the accessories off the shelves – now I just apply the Velcro to the bottom of the accessories’ feet instead of removing the feet, since the accessories (including the switching PS) are much lighter than the rig and are easily held in place that way.
In any case, since the Velcro stays on the 6300 case bottom and on the shelf, in the future to go portable again all you have to do is remove the feet and set it on the shelf.Howard
It isn't all that hard to have straps added to a shelf to hold gear that isn't rack sized.
Roadies often secure the gear this way, even if they also build a cosmetic set of 'ears' or a filler plate to give the appearance of a faceplate rack mount.
I've seen a neat audio setup that used fabricated "pockets" around a non-standard sized processor, which the roadie used a sort of glove-box for accessories.