On my first couple bikes I didn't have any place for my insurance papers. I didn't have a panel I could use, I didn't have any storage under the seat.
I didn't want to use any detachable magnetic tie-on soft bag so I opted for something secure and lockable. The larger canadian plate won't let it open all the way without modification but if you have a bike and no place to put your papers here's a better option than your pocket.
They've been around forever, can be ordered in to several bike shops, and should run between $20 and $30.