how to mount a subwoofer for deeper sound

first thing to ask is what subs are u using????

next thing is have u considered going vented?????

Use a lot of sound damping material.  The rear deck should be covered with it, anyway, whether the sub is mounted to it or not.  I would suggest keeping it in the 1 cu ft sealed box and continue working on placement and damping the sheet metal.  It's still going to sound like a budget sub (I still use one of those from about 4 or 5 years ago and I think it sounds pretty good), but it shouldn't sound bad...and the car shouldn't be rattling.  But at the same time, it's not designed (nor priced) to be a strong contender much below 40 Hz, so keep that in mind as you try to get the most you can out of it.

If yours is just a badly made sub it should be replaced.

Oh yes, and this:  keep trying also to reverse polarity to the sub every time  you change its position, then give an extended listen while sitting in the driver's seat.  It's easiest to reverse polarity at the amp terminals.  While you may get deepest response with normal polarity in one position, you may get better response with polarity reversed after moving the box to another location or turning it.