This is an idea I've come up with for finding partners more easily. It might work. Or it might not.

If you want to play a co-op custom map RIGHT NOW, try entering this group's chat room. If anyone else wants to play at that moment, and they've also read this announcement, they will be in the chat room as well. Then you can add the other person to your friends list and organise which map to play, who will host etc. Leave the chat room before you start playing so that other people know you're now unavailable.