Ok first off... I don't know how old you are or what qualifications you have etc so I'm just judging based on the image.
The letter, a cover letter is normally quite formal in it's appearance and approach.
Now the design on the other hand and this is likely how apple design staff would see it. They who will likely have 100's if not 1000's of applications too so you need to stand out.
As a qualified product designer who works in the field on a daily basis I'm just not sure what's so special about that design. It doesn't have a pleasing design to me (it does look like a bread bin as mentioned), it's much bigger than the ps3 and wouldn't fit in most people's home decor theme.
From an engineering standpoint (we had to do a basic level of this too on my course), it's not really that impressive either, it's literally a few cut blocks of plastic (likely planned in software then machine cut) and a 'curved' piece. The metal sheeting on top looks like the cheap stick on stuff you could get for cars although this could be down to the image and your screws could have been counter sunk, there's enough depth in the plastic sheeting.
You say you've focused on cooling the ps3 yet you've bought off the shelf parts which don't take any engineering skill to put together (most of the hardcore pc users on here can do it) rather than showing off your engineering skills off and made a custom heatsink to aid cooling. Not to mention Apple internals would never be that untidy.
If I'm completely honest, I'm just not sure it's up to the level that Apple would be looking for, they can literally pick and choose the best of the best.
