After exploring battery relocation ideas with this forum, and some of the members even involved in this thread, i decided to use 0AWG ground wire to one of the seatbelt mounts under the back seat. It's non intrusive, it was easy to grind paint away, and i should not have to worry about corrosion or rust. It's a nice fat cable, with a nice clean, large contact patch. I'm not the least bit concerned it won't work properly. My last relocated battery didn't have near as good a ground, and i never encountered any issues because of it.
My battery does die a bit after a couple weeks, and i have to put it on a charger. But that's because i keep the clifford alarm armed 24/7, and the car is sitting in the garage without being driven or run.
Having said all that...
Rather than argue about what's the best way to mount a battery in the trunk, how about addressing the original concern?
OP has a car with the battery in the trunk, and it won't stay charged.
Okay, now let's not assume it's simply because of the location of the battery, or the ground, for that matter. Now that the grounding point has been beaten to death, let's explore some other ideas.
Is the car driven often?
Does it have an aftermarket alarm, or any other potential power sapping devices installed?
Does it discharge even while it's running, or just after it sits for a while?
Do the headlights dim significantly when the heater is turned on full power?
Do the headlights dim when the turn signals are flashing?
Has the charging system been checked, at all? (VatoZone does this for free, as do other parts stores)