For 25 years all my cars have had  trunk mounted batteries. That's one
of the first things I do to a car, after brakes of course.  I use a
simple marine box from Wal Mart. DO NOT use those little plastic strap
brackets, they get brittle and break, leaving you with an upsidedown
battery leaking on your luggage. I either screw to the floor two metal
brackets for the strap. or mold in a loop-type of shape into the
fiberglass when I am building a "glass" floor, and loop the strap
through that. 

   For cables I buy replacement welder cable (they are very flexible,
made for high current,  and made for rough use around dirt and grime).
of a size almost twice the size of the typical 12v battery cable. I run
it along the side  of the frame rail. And-  NEVER NEVER NEVER run a wire
of large size OR carrying a substantial current alongside, or crossing,
a fuel line or brake line.  Car batteries make exellent welders,
especialy on metal tubing, if you ever get a hot wire, a nicked wire, or
a rubbed wire. How many of you hang yor tailight wires on the nearest
available brake tubing for support ???  Also,   Remember the voltage
drop inherent in a cable that's 15 to 20 times the length of the
original: Use big cable, you'll regret it if you skimp!!  I ALWAYS
solder the lugs (welding supply) on the cable end, even if it's crimp-on
style. 

   For the openings the cable passes thru? I use, I forget the name,
those fittings that electricians use to pass wires through panels in
breaker boxes. They come in many sizes. For a nice look, find one that
looks right for your cable (not a big hole-little wire: ugly). I like to
put the clamp-side under the car, with the nice nut-side showing in the
trunk. If you don't like the look of the star-shaped nut, you can use
instead, a plastic end (I forget the name again) that electricians
thread on to the ends of conduit to protect wires from being cut by
exiting the pipes.  You can seal it by forcing a bit of silicone sealer
around the wire, then wiping it off with a paper towel. You won't see
the sealer.

   For the corrosive fumes and drips, I found in a J. C. Whitney
catalog, actually it was from a Warshawski catalog (remember those?) (a
JCW company) about 20 years ago, a battery vent kit that replaces the
press-in battery vent-lid on almost any battery with a closed lid that
vents through a tube.  When I buy a new battery, I transfer the vents to
it. Not helpful??  In 1985 we bought a new car. The Ford Tempo with
diesel engine had a large battery in the trunk. Ford (and replacement
brands) had a battery that vented through a tube. They also had a very
good "bag", vented with a hose thru the floor that was then covered with
a box-shaped decorative cover. I never had any trouble with leaks,
smells, or deteriorating carpet, etc.  It was a very effective system.
(diesel- 48 MPG!!)

      I always mount mine over the rear axle for the best use of space
and less bounce than other locations.  Many advantages, especially in a
light-in-the-tail Avanti. Put it near the right rear wheel. The only
drawback ( also an advantage!)--  the battery never gets engine heat. A
cold battery charges slower in winter weather, but no problem unless
your batt is marginal. Engine summer heat helps deteriorate a battery.  

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I'm following most of your suggestions already, with the
exception of using rubber grommets for the battery cable
through the floor point.  I will check out what's available to
replace those from an electrical supply house.

I'm using 1/0 welding cable, which should support the engine's
starting current requirement without too great a voltage drop.
The positive cable will be about 16' long, the negative about
6', as I want to ground it to the left frame rail, then ground
the engine a second time between the left frame rail and its
normal grounding point at a water manifold hold down point.

I'm not using a box at all, as the battery will sit on the rear
shelf of my GT, right behind the seat back.  That entire area was
encased in foam years ago to add rigidity, so I cut a section out to
hold the battery.  I need to create a support across the front to
keep the battery in-place, and some shims on top and sides to 
do the same.

I'll check Whitney for vented sealing caps.  

Thanks,

Chris
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