I have a lock box that secures to the seat post via a cable. Similar to this one.
I have a couple of these in my vehicles and they work great. I also have a larger version I use if traveling for a few days or longer where we may have multiple handguns. Consider a second for ammunition since some states require the firearm and ammo be in separate boxes and I prefer separate lockable boxes.
Stay away from fingerprint / scanner lock boxes as they are not reliable. Wet, sweaty, dirty, not aligned, etc finger are just a few situations where they are known to fail.