I was using my own tray that I cut out an old ATX case (actually chopped into 2 pcs) to match up to the existing 4 PCI slots in the G5 Case. I suppose you could do the same with the MM tray. The method I used permanently and securely attaches the tray to the back wall of the case.
I adjusted the height off the back of the G5 with numerous small wafers of aluminum plating (less than 1/16" thick). This allowed me to test by installing PCI-E cards for proper height fitment. Then when I had the right height, I JB Welded several stacks of the pieces to the rear of the motherboard tray and let it harden for a day.
When installing the tray into the case, I attached the motherboard to the tray, then JB Welded the wafers to the case, installing the PCI-E cards at the same time for an absolutely guaranteed fit while the JB hardened. With MB and PCI-E cards in place, I place heavy weights onto the assembly while it hardened to the case wall. I left it for 24 hours.
After it hardened and I removed the hardware, the result was a perfectly aligned motherboard tray at exactly the correct height to match up with the PCI slots. Its not elegant, but would later be completely concealed by other plating and the MB. You can see in the pic the 8 required brass standoffs in perfect and permanent position for my micro atx board. Oh, and the best part was it was free!
View with Motherboard and PCI-E cards intsalled...
Something like this method might work for your MM tray, I'm not sure. One thing for sure, its not gonna move once its in...