Moved to Audio & Security.

When I bought my single DIN headunit from Crutchfield, they included a panel kit that mounts the deck at the top of the dual-DIN opening, and has a small slot-like trim panel underneath.

Because of the bulk of the wiring harness, I haven't reinstalled my stock pocket. For me, it's no big loss. I'm more concerned about securing the headunit anyway.

Look around on the net and you'll find a range of dual to single DIN conversion kits. You may need to do some additional fabrication work, but the retail kits are a good place to start.