Plastic might not even be strong enough since these are so thin.
Painted then, or anodized white (is white anodization possible?).
Aside from the potential stability concerns already mentioned in sibling comments, the aluminum case is essential for dispersing heat, as the MacBook Air relies on passive cooling instead of using a fan.
The plastic Macbooks had a lot of problems with cracking, wear, and discoloration. I'd be surprised if they ever came back.