Those plastic things also come in opaque. Or you can get contact paper and put that on. Just don't get "girly" stuff.
Does this landlady go through his drawers and closet? If so, then you have a bigger problem then her just finding out about you. That would be a total invasion of privacy - likely illegal for any landlord to go through a tenant's personal things. Why does she show up out of the blue? What, do you think, is the purpose?
Ideas off the top of my head is keep your personal female items - like hairspray and makeup out of the bathroom. Put in bag, zip up and keep under sink. Keep a "girly" bathrobe, pajamas, nightgowns put away. Not hanging in bathroom or thrown on bed or other furniture. Don't leave laundry sitting out where female clothing can be clearly seen.
A bigger suggestion is find a place you don't have to hide.
Swop clothes storage with boyfriend so HIS stuff is in the see-through boxes. While the landlord has the right to inspect the flat regularly, or enter in an emergency, turning up out of the blue is not acceptable, this does not give your boyfriend the privacy to which he is entitled. He should ask her please NOT to come without giving advance notice.
Most people don't like visitors coming out of the blue when they haven't had time and get the place clean and tidy.
I've been living with my boyfriend for the past 5 months but it's a privately rented flat and I'm not supposed to be here, and we'll be in trouble if his landlord shows up one day, which she could and has done before just out of the blue, and when that happens we have no time to go and hide my stuff. But I need more storage to put my things, I can only really afford those see-through plastic draws, but I need a way to make them non see-through so she can't see my stuff in them. Any ideas on how I could do this or an alternative?