Diana:
That's a fairly simple setup, but it has some drawbacks.
1. The 6X9 speaakers don't seem to be fully baffled, that is, it appears that the rear part of the speakers is just open. I'd recommend putting hte speaker in an enclosed box, for best effect. (This can certainly be done with similar looks, you just need to have a box underneath.
2. For that kind of setup, I'd probably use 1/2" plywood for the board. Inexpensive and simple. The padding and upholstering will make the wood type relatively unimportant.
3. The cargo cover's mounts are visible in the top picture. There's nothing in them, so that's been removed. It does look, however, as though it could be reinstalled, and it would just go over the speakers.
I'm not a fan of rear speakers, particularly not in this car. I spent a lot of time tweaking to get them to sound good, but found that things sounded by far the best when the rear speakers were just removed. Your opinion may vary. 6X9s are nice because of the large cone area. Coupled with a good enclosure they can get decent bass, rivalling some 8" woofers in some cases.