Here is a sweet 540 to 380 aluminum adaptor plate
It's fairly thin, plenty of holes to choose from (in case one gets stripped), perfect machined quality, motor won't slip out of position, and transfers heat much better than just using a 1 screw method. I haven't tried it with brushless yet, but I did try it with a 380 Mini-Quake motor, and it worked perfectly.
from Germany makes the adaptor. The USA distributor is finedesignrc.com
. You'll have to call finedesignrc and ask them to get it. They call it a 'Distance mounting plate for 15er LMT motors'.