Autozone has a line of sealed performance batteries and have one that fits e46's perfectly without any mods. $185.00 usd.
Here's the stats
Here's the way it looks mounted
The only difference is the height by about 1/4 inch or so, but the bracket that holds the battery has a screw that you can adjust for that, here's the screw
Loosen that screw until the holes line up, I tightened the one closest to the spare wheel and then lined the other one up with the adjustment, perfect fit. Don't forget to tighten the adjustment screw for a tight fit.