I made my box with no vents all sealed up and has been up there since May 2004 with no problems.

1) Open box with one gland for pigtail, the other one for cat5e isnt on yet.
2) This is how the box looks closed.
Here is a different completed box out of the bulk i assembled.

The box went up and has kept my Minitar safe since May 2004
There are more images on my site if your keen to have a look (i should update it sometime)
I did a bulk buy on boxes and glands and almost sold out now. I use the ABS plastic ones, they resist the suns rays pretty well.
The people down the street form me put a Minitar in a box (not one of mine) and sealed it up but put one of the glands on top so when it rained the box filled up and the minitar was sitting in about half full of water, It still works perfectly. So put your glands on the bottom or sides!