You're probably not going to find a very small 2ch amp without paying big bucks.
Unless car audio has changed since I last nerded out on it, 2- and 4-channel amps are still using a Class A/B switching design, while a lot of sub amps are Class D (smaller, efficient). I believe the Alpine PDX series amps are Class D digital 4 channel amps, but aren't cheap.
I wouldn't mount an amp in the glovebox or under the seat. It's just bad for ventilation. The best possible place for an amp is laying horizontal, in as open a space as possible.