There's a little room behind the battery (sort of in front of the brake booster back about level with the EGR valve) but it's close to a lot of heat. Might not be the bestest place in the world for it.
I'm building my own power distribution panel and my intention is to drop it between the headlight and the battery. This will feed power to the headlights, 3 sets of driving lights, 2 sets of power feeds to the rear (tub and tow), electric brakes and my new air horn (no more girly horn for me!). Things should be a little nicer to electronics in there.