I want to add to Kabouik: the N900 keyboard was good. I'm not very familiar with hardware keyboards on phone, I had a detachable one for the Galaxy S3, which made very hard clicks. It wasn't perfect, because it worked on BT and switching it on/off took too long to have advantage concerning typing.

The strong point of the N900, and if I can remember it has been mentioned in one of the reviews was the keys weren't flat, but concave, so keys next to it weren't easily pressed. I also like soft keys (no CLICK, CLICK!, CLICK!!) and a very soft backlight, and even better: switchable. A hard backlight can be just as bad as no backlight, because I don't want to lit the bedroom when I'm typing in bed.

Concerning thickness I want to say:
I would try to make it thin and use metal. It's good to make it a bit heavy, so the keyboard/screen are balanced a bit. Using metal can also make it stiff. To make the design more easy, you could try to build it in two parts: a "drawer", where stiffness and a good sliding out is important (not angled), and the real keyboard part (screwed upon, thus repairable?)

Another simple yet effective design (I've made it once from plastic and glue) is making a room with four "walls" and making an "X" in the middle.
I've graduated for mechanical engineer, so if you want a good design regarding stiffness/size, maybe I can help. robnas at gmail.com (dutch).