When I got a case for my MacBook (same Alum model as yours), I opted out of getting the Speck case because for one, it is $50 (which is more than twice as expensive as the one I purchased), and two, I feel like I would have been paying for the name brand more than anything. Also, I am not very concerned with damaging my MacBook because I don't bring it with me places very often (home and work), as I do much of my developing work at home or at the office.
This is the case I purchased off of eBay. After using the case for a few days, I noticed having the bottom piece of the case attached held the heat in more compared to when it was not attached.
Other than that, the top sleeve of the case looks great