The problem with using a computer case vent and making a replacement window is closing the vent at times...
That Van Air vent looks awesome!! It closes up tight, and is fairly compact. A 20w solar panel would run it fairly well too, so you could just leave it open and on and it would run during the day w/o draining the battery. Be great for moving a little air through a truck topper when camping, or just parked in the sun.
As for cutting a hole in the roof, I wouldn't worry about it. A small round hole is certainly more structurally preferable to a large square one, and easier to cut too. Just use a sabre saw with a metal blade. The Astro has a nice flat spot on the roof that's made for a vent, so you could just center the round one in the square vent spot and pop it in.
If you do end up using one vent, let us all know what you think of it!
Chris