Polyethylene is see-through and water resistant, but attracts dust and does not filter light. Muslin or cotton sheeting is lighter, cheaper, provides light protection and can be washed when it gets dirty. It does not protect from water leaks. Methods of attaching dust covers vary. It will depend on the skills of your staff and volunteers.
Some museums use velcro, others use ties.
Covers can open with tie-backs, roll up on a rod or pull up like blinds. Slotted shelves vertically store small to medium-sized framed artwork in good condition. Usually constructed by the museum, these shelves have vertical dividers every ten to twelve inches. Each slot holds two framed works. Frames are stored vertically, their full weight on the bottom of the frame.