What are you guys doing to set up a stable position when shooting out of a window or off of a barricade about waist high? Do you use rearward pressure with a bipod in front of the barricade, a sling, or a bag on top of your knee/leg, or any other methods?

Gamechanger bag, or whatever heavy bag off your choice, let the rifle balance naturally and I use my left arm to push down on the front part off my stock.  I use a modified form of free recoil so i'm not really pushing into the gun with my shoulder, but not totally letting it slide back after a shot.  It takes a little bit of getting used to but its not all that difficult.

Some guys grab their scopes like in this pic, but this is how most people approach that sort of position.

I appreciate it guys, that's extremely helpful. I'll give it a try

You just have to find what works for you but the biggest thing is a bag, liker the game changer, or solo sack.  Something with a heavy fill really helps and lets the rifle nest down into it.  Anything positional I flop the game changer down on and go to work.   There are times where I use that over a bipod if I need to move a lot on the clock.  I might sacrifice a little in stability but make up for it in being able to get through the stage and not fight a poor set up.