The PO of my van paid a shop $1200 to install a really nice system in my van...
The shop decided to mount an amp under the passenger seat and to wire it up to the battery (the starter battery).
duh...
my plan is to keep the setup the same, except to swap it over under the driver's seat and wire it to the aux battery.  Anyway, it's an amp that's a bit too long to fit in the space flat, so it stays angled up on one side.  This is actually good for the cooling, as it gets airflow on both the top and bottom.  It stays in place pretty well, and is a good use for the space.  I've had no issues with overheating or rattle...or power draw for that matter.

The Eclipse head unit they installed is actually really great, as it has a feature of adjusting the front panel angle...so I can angle it up for better viewing from the typical driver's position.