I wish I had some skill to mock up my thoughts on this, but isn't that was pscates is for? :-)
In lieu of that...
Add enough to the dimensions so there's room for a slot-loading drive, but otherwise slim down the borders as much as possible, and of course, use decent materials since this is a Mac, and not a VAIO.
The other option would be to keep the trackpad where it is on the MacBook, but angle the palmrest down, forming a wedge without the keyboard having to be at a particularly steep angle.