When I added 2 amps in my trunk I knew I'd have to drill into something to hold them down - most likely metal. I never liked doing this. So when I installed the amps, I cut a 1/4 piece of luaun board to fit the sides of the trunk. I slipped the board under the carpet, put the carpet back in place, held the amp over it to mark my drill points and I drilled just down into the board. The carpet with the board under it now hold it firmly in place.

 

lucid,

One trick I use to hold things fast without drilling/cutting... I glue small pieces of plywood/particle (liquid nails is excellent and can be removed later) board to the area I would use a screw... and screw into the small piece of wood instead of the steel...

Later, the piece of wood can be knocked off and the small trace of adhesive can be removed with a chemical glue remover...