They seem to be very popular. and if you want one- I might become your best friend... I have one that was installed on a buggy- but easily modified to fit on any other vehicle (Im actually using it on my savage). This design can fit up to a 51t spur gear without modification-aside from clearancing whatever plate it is mounted on.
I plan on mimicking this mount (after I get the approval from rc-monster mike). I will be using 1/4in 7075 aluminum stock and a cnc mill. By the final design expect to see more pockets for recessing and weight reduction. maybe even a unit to house larger spur gears... we will see.
Here is the mount that i have in my shop
Here are some of my original CAD design- updated one coming soon
Please post thoughts comments and concerns