Now that you're done, how did you end up mounting the rear of the sled? I was going to suggest what I did on my Toyota pickup (I hope you had more room to work with than I did!). A few inches above the rear of the sled was a round steel tube - some kind of structural member. I looped a length of flexible metal strapping around it (the kind with holes along the it), then placed a clip-on blind nut onto both pieces of the strap hanging down. Drill a hole through the sled at the apropriate spot and bingo. I originally used the threaded hole in the sled, but that was *exceedingly* difficult to do. When I had to pull the sled out, I thought of the blind-nut idea.

About you original head unit. On mine, it was an AM-FM only cheapo. I found I could disconnect the front of the unit. It all came off as one piece except for the LCD. So I placed a piece of greenish-colored paper (cut from a magazine) behind the window and affixed it with tape. The knobs are held on by screws. And then it slid into the sled just right. Push on the right side, and left pops out to remove it.
Not a perfect stealth cover, but at a cursory glance, it looks like just an AM-FM is installed.