I don't think there's room for a cap in there. I would avoid a car wash that puts that much pressure on the charge door...I can't imagine what that's doing to your paint. I go through an automatic car wash at least once a week and I've never had my charge door pop open from the brushes.I recently got a 2017 Volt, last Saturday I went to car wash. The automated brushes actually popped my charging door and I was freaking out as this was the soap cycle. Long story short, it's working ok but was wondering if a cap is available to avoid this in future? All my searches just lead to door cover replacements.