We are just wondering WHAT WE NEED TO ACHIEVE THIS.
A deep understanding of thermal physics.
Depending on how high you want to go, on the way up things will get very cold, so you'll want good thermal insulation, but go high enough and the lack of air density means convection cooling doesn't work and your PC will go into over temp shutdown.
But if you can get data transmitted real time or saved to the HDD, just let it go until it starts failing. You can still get some cool pictures. It will work for a while...it's just a matter of how long. Then there's the matter of finding it when it comes back down. You should not count on the GPS unit functioning as it comes back down unless you really design it properly (active cooling and heating and humidity control) (and radiation shielding maybe too).