Am I just being an idiot here, or is there a reason why none of my custom single player games save? I thought it was supposed to automatically after each turn, but even if it doesn't, I still can't find a save button anywhere. Halp pl0x?

There is a 'saved game' tab in the sub-menu to the right of 'Single Player Main Menu'.

Click that during a game to save it and activate autosave.

Ooooh, thank you so much. I never did that because I thought it would exit my game and bring me to a different page.

Understandable. Note that once you do so, there will be a grey tab at the top to the right of 'Help' that will bring you back to your last game. This only works for one navigation away from the game to my knowledge so don't stray too far without saving.