I use the Deka ETX30L here:
http://www.dekabatteries.com/assets/base/0278.pdf
I keep it on a Shumacher charger (much cheaper than the Battery Tender brand), and it has been very reliable for about 2 years. At 22 lb, this one is not the lightest around, but definitely better than OEM.
I have mine mounted on its side. Any of these will be a lot smaller than stock, so you probably need to have the ability to fabricate your own battery tie-down system.