JoeAirPilot,

If the heat inside the cowl is such that it might hurt a battery, then it's way too hot to run. The engine would seize long before the battery felt any strain.

And shawns method (the same as I used) is to strap the battery to the underside of the the engine box top. The air pressure will draw air out the bottom of the cowl and not really let much heat inside the engine box or fuse.

You can feel very safe putting it there.

The only thing I'd see wrong with putting on the firewall is lack of area to adequately pad and isolate the battery from vibration. You don't want it inthe bottom area as it cna restrict airflow out the cowl and indeed cause a higher temp... again worse on the engine than battery.