If you're looking at regular matchmaking, you're best bet is a giant ego and somewhat decent forging ability. Once you make a decent map begin advertising it and test it to death in customs. Make a thread on bungie.net halo waypoint and get some friends to help you bump it to keep it on the front page. Eventually these people called the community cartographers will notice your map and put it in matchmaking.
Make friends with cartographers, and you can get any map in matchmaking like Unconquered or AgentPaperCraft's map.
If you want to do something via Forgehub there is the "library" section designed for that. -Info -Place to submit maps