On any map you can Forge, there arewindows and chain link fences that you cacn go through, such as Foundry and many other maps. With two players, ahve one player make a machine gun turret and set it down, then have the other person will get in it. After the person gets in the turret, have the other player pick it up with the Monitor while the person is still inside. Move it to a chain linked fence or window while pushing the player until he goes through the obstacle. Have the other player turn into the Monitor and go back into player mode. Do this when you see the window or fence in front of your face. Then, usually, you can move around and whatever desired. You can also try to put Spawners and Teleporters inside so that others can join you.