If Mike can't get the motor mount made to your specification, I've been using various angle aluminum for motor mounts with good results. In my Jato (Mamba Max 4600) I use 3mm thick, in my Revo (Wanderer 10XL) I use 5mm thick. For that motor, 8 or 9mm is definitely needed. What about
this (6.35mm) or
this (9.5mm)? Of course, you'd have to cut it to size and trim the excess for weight.