I can highly recommend the Sony CMT-EX1 desktop CD system. It is a shelf system not a "boombox", it is slightly over budget(around $299). You will be surprised of the sound this little thing can produce, it has a tempered glass front plate that you can see the CD playing(which makes it verticle loading). I have this unit in one of my garages and it holds it own, I am not sure how it would do in an uninsulated garage but I imagine it's performance would not be changed for years.
Cheap system for garage
A friend of mine asked me to recommend a system to place in his garage. The space is uninsulated and located in the New York metropolitan region. He will be listening to CD's while working in the garage. I thought that what really makes most sense is for him to get a $200 boombox rather than spend any kind of real money on components that may not tolerate temperature swings. Any other ideas? Can anyone recommend a decent boombox?