1 - If your scared of your landlord coming by and looking around then don't grow in your place that your renting because they can make different excuses to come in and check things out
2 - If you want to make things more water proof use one of those containers that they use for the flood tables if that isn't an option then get some heavy duty plastic, tuck tape (the red kind of packing tape very good stuff) and build a part one. the other option is to buy some plywood and 2x2 or w/e screw them together to make almost a tub like thing prime it to seal it then use cocking (non toxic) on the seams to seal it even more then do the plastic thing to it and use the tuck take to take the peaces together on the inside and out side.