Most optimal "hidden" portable Steam client?

Hello!

To sum up a long story, my gf's very traditional mother really despises when she plays games. So, until she moves out, do any of you have any tips on keeping steam portable?

Obviously a portable drive, but more so asking about specifics, like if a thumb drive beats a larger one, or if we could hollow out a book and hide it in there. Looking for real world stories here. Also, bonus points, how would I go about keeping files away from say User folders and app data? Don't think her mom knows how to get that far, but we're practicing safe gaming.