My ETX900V is mounted aft in my 8 and is vented through the belly skin behind the aft cargo bin. I used short lengths of 1/4” tubing as exit ports which are attached to a piece of angle. The angle is glued to the interior of the belly skin and serves as an anchor for the vent tubes which are cut at an angle flush with the bottom of the skin. I’ve not experienced any issues with engine exhaust entering through these homemade vent ports.
I’ve just started construction on another RV8 and intend to use the same battery and mount it forward in the same area you are. For now, my plan is to use one of Earthex’s vent ports and either mount it in the bottom skin just forward of the right main gear strut mounting bay (abeam the exhaust ramp), or perhaps mount it on the lower right side of the fuselage (which probably would be a bit unsightly.) I don’t think it’s necessary to put the vent port in the back and run vent tubing aft under the floor the length of the cabin for a forward mounted battery.
One consideration is if you intend to install a smoke system in your plane (as I have) is that even with check valves in your smoke oil lines, you most likely will still get some residual oil running down the belly of your plane. Any obstruction (e.g. an antenna) or exit port will gather oil (and dirt) there which, if not kept clean, could potentially block the port.
Regardless of what you do, I would follow Earthex’s recommendation to vent the battery directly to the outside of the plane. I would not rely on cabin airflow alone to vent any toxic fumes in case the battery had a thermal overload.
Regards,
J Baker
RV8