well you need to find out what is hot first before you can cool it
my guess is that its may be the processor or something. since your going to take it apart anyway, whip the lid off and play some.
have you got an infrared thermometer? since its for A-level it will be good using somethinging like that to pinpoint exactly what is hot and document it with the lid on, and then with it off.
then what you will need to do is assess how you are going to make something to keep that area cool.
if it is just the processor that is hot, then a good idea may be is to take off the stock heat sink, and make something similar but improved.
what would be good is to research heatsinks for processors and document your findings. make sure you find out the mechanics of why they are made that way, and with what materials.
if it is the processor (it might not be) then an idea might be to have heat pipes coming through the case at the back, with some big fins like what a thermalright looks like, with an ultra quiet fan sucking air from it and blowing upwards (cos heat rises)
god i must have took the wrong subjects at A-level.. i completely flunked it!
why have you picked an xbox anyway. why not a pc - so much easier to do this. but good luck