PSA got me again with that 16" Mid-length upper with M-Lok handguard for $219 today.

This will be my first AR with any type of rail. I want to mount my Surefire 6P to it but I want the ability to quickly remove the entire mount and light. I want something secure but lightweight.

Any recommendation for a simple mount that is easily removed?

An easy and cheap way to do it would be with a short section of rail mounted to the hand guard and then mount the light directly onto it with a 1-inch weaver style scope ring. Unscrew the scope ring to remove the light.

You could look at Kinetic Development Group Kinect mounts:

https://kineticdg.com/product-category/kinect/

I've got my Inforce WML mounted on one and it's seems to be pretty secure and easy to remove from my rail.