Interesting thread. I had planned using a battery tender from November to late March for my Genesis G70, but found out the battery is in the trunk. First car I've ever owned with a battery in the trunk. So running the extension cord became an issue as I didn't have any holes to run the cord through. I really wanted to use the battery tender, which I ended up returning. So now every 3 weeks I remove a portion of the car cover, climb in, start the car, and let it idle for 20 minutes. I vary the revs during that 20 minutes. I changed the oil before storage and added Stabil to the fuel. So far so good.
Battery is in the trunk on my cobra. I just let the cord run out and lightly set the trunk on it. Zero issues.