Maybe it has been asked but how can you master the current horde map? Matchmaking is inconceivable only. And it is not available in custom lobbies.
It’s coming to Custom today. TC either forgot or had a bug that didn’t make it appear there.
Then try to Inconceivableing it before Mastering it!
Already a thread for that Rail line not in Custom Horde
TLDR: Should be fixed today