Because the stock radio sits at a angle and mates up to the trim pannel I decided the best way to ensure a clean fit was to use the same mount points.
I found that Metra makes a DIN adapter kit for the 08+ Impreza for mounting a double DIN HU into the stock non-nav bezel. It was a modular kit with mounting brackets that were separate from a trim mask piece. I ordered it just for the brackets. Now I needed something the size and shape of a double DIN HU to attach the brackets to so I built a dummy double DIN head unit from sheet acrylic. I left the back open so I could mount accessories like a DAC or USB hub, right now it just has a lighter socket and power adapter.
Building the dummy DIN
Trimming the brackets from the Metra kit
The brackets attached using machine screws and nuts from home depot.
The assembled unit with the brackets on and everything mounted.
I wanted to be able to remove the tablet from time to time in order to work on software, do firmware flashes etc. I decided to use a snap on back cover from ebay to actually hold the tablet in place. The snap cover is then attached using sugru to the acrylic head unit dummy. That gives the mount some flex to absorb road vibration.
The snap on back cover adds a bit of thickness to the edge.
The whole rig with tablet removed showing the back cover used as a mount.