ORIGINAL: cmoulder
Alternatively, you might just get rid of the X mount altogether by making a triangular ply sandwich and mounting the motor directly to the ply using the three small bolts that attach the motor to the X mount. I did this [link=http://www.rcuniverse.com/forum/fb.asp?m=10361753]on the Sickle[/link] and it is plenty strong and weighs a heck of a lot less than the X mount with standoffs/bolts/whatever.
I used a similar method on a converted Brio as per the recommendation of Jarvis of Maxx Products who imports these motors. It allows for the use of the supplied "cooling washers". The "X- mount" was totally eliminated as per his recommendation. My firewall is thicker (ACP Composites 1/4" end grain balsa with carbon fiber laminates on both sides) . Be certain to cut open air through passages that correspond to the motor and light ply donuts ( a bit larger than the size of the cooling washers ) beneath the cooling washers on the back of the mount.
Another trick to keep the laminate from crushing is to insert a length of alum. tubing that just fits over the motor bolts as a sleeve through the firewall. CA them in , making sure that they are no thicker than the width of the mount.
This setup has worked without any problems on the Brio and I have also done the same on my yet to be flown Griffin. That'll be held up for awhile because some other pilots have lost aircraft due to a wing structural problem.