Our airframes are not composed of Aluminum Oxide. That would be non-optimal if they were. (see
https://en.wikipedia.org/wiki/2024_aluminium_alloy)
In my "opinion" , the airframe makes for a fine ground path. Ohm's law would suggest that they are capable of carrying many many thousands of amps. Real world demonstration of lightning strikes on aircraft in flight back up this assertion.
For airframe grounding; Run the braided strap (1-2ga equivalent) from the battery [-] to the longeron. Use an AN5 bolt, lock washer - internal teeth - MS33353, AN960 washer, K1000 plate nut as the attachment stack up. Apply a thin coating of petroleum jelly (wipe it on/wipe it off) to the connection -- this serves as the gas tight barrier against oxidization.
On opinions:
For grins and giggles, measure the resistance of your airframe -- A point on the firewall to any point on the aft bulkhead. What did you get?
I measured my completed RV-7 -- it was 0.27 ohms, the in progress RV-14 was 0.3 ohms. I used exposed AN hardware, finished holes, and random points on the skin as contact points...