Well doing it in the loo is always an option, if not the sexiest of choices. But the main option is learning to do it quietly while in bed. You should be doing it each morning and night in bed, as well as your afternoon one.
I also think you need to worry less about your roommate. Certainly for guys sharing rooms it’s kind of a given that they may have a crafty wank sometimes, and it’s a lot messier for us… If they suspect you of doing it, it’s not the end of the world.
Of course, the optimal solution is they become denial buddies! Leave your laptop or phone open with an intriguing post maybe, or get tipsy and engineer truth or dare that gets you talking about it. ‘What secret kinks do you have?’ for example, and then they’ll ask it back.
Then you only have to worry about them hearing you in the next room…