I've finished a map that themed parallel to the odst layout with a few twists if anybody wants to check it out lemme know. GT chiefymaster117
I suggest submitting it to MaxExtras testing lobby. Coordinating a BTB playtest with the low player count at this point is pretty challenging.
Best way to get a full lobby is play a game of btb or 16 player game in custom browser. Once it's over invite everyone. You may only get 3 or 4 but this may be enough to get the lobby going once you start the game. As dipper says it's tough to get a btb lobby these days but testing lobbies like Max's are a good way to go.