Mine's built into the box, was feeling lazy when I bought it, so got the Vibe box with built in amp.
You could mount it on the side of the box, or anywhere in the boot.
Depends how big your amp is - if it's a massive FK off 4 channel amp, it will have to go in the boot.
If it's a 2/mono channel amp, you probably won't get it in the glovebox.
You could have a look at trying to squeeze it under a front seat.
Apart from that, I can't really think where else...
Websites are like ********s, everyone has one, and im wary of any bloke trying to direct people into their's