First play Construct on Forge, follow the same steps you would use to get out on Forge. Play as Spark and hold the left trigger to fly down the purple lift until you reach the very bottom and return to normal. After that make a Two way Node and place it on one of the walkways that elad to the glowing circle in the center of them.

After that place a Two way Node anywhere else you'd like that can be reached without needing to be Sparky. Then when you play in Custom Games you can step through the two way Node and you're out of the map.

You can also place weapons out on the walkways outside of the map so you can use that as a way to gear up before reentering the map without committing suicide. I suggests using a two way Node so you can get back in and keep killing... good hunting!