There is absolutely no reason why you couldn't mount a grip under the main upright grip on the mini rover. You need the upright grip portion to mount the sound gear so that would stay.

The aluminum bar on the mini is thick enough to drill and tap. There are many grips that are available with male screw threads.

I tried the mini as you mentioned but there isn.t as much regidity using the grip on the bottom.

You want cheaper
http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=131401&is=REG

I had to shift my mounting point forward of the bracket itself, not just side to side, so using an additional plate as well as the long slot on the flash bracket worked well for me to get total adjustment flexibility. I think it's safe to say any of these could be used and modified with great success. I certainly know that using any kind of vertical handle in addition to gripping the camera with the right hand is a huge benefit.

can you post a picture of the one you built?

matthew