I keep my Kryptonite U Lock underneath the passenger seat. It fits perfectly. It's easy to get in and out. I use velcro to attach it on the part of the lock closer to the front of my bike. It allows me to keep a serious lock with me with minimal hassle.
There are lots of people here with more experience and opinions about what is sufficient security in different contexts.
Here is my tip as a Rad Runner Plus owner. I bought one of the oft-recommended Kryptonite U-locks (currently on sale). I got the 11.5 inch lock, and when it arrived I had all kinds of buyer's remorse that I had spent $100+ on a lock that was too big and would be a hassle to carry on the bike. My partner suggested storing it under the seat (see sketch), and it fits there perfectly! It's as unobtrusive as a serious lock could be. It's easy to stow and retrieve, and I love how low profile it is.
I use a piece of velcro on the front of the lock to secure it. And, in fact, having the extra length of the 11.5" lock has proven useful when trying to lock up in various locations.