People use scrunched up balls of paper to reduce the open areas of an enclosure for snakes that seem particularly stressed by space. It's most often done in enclosures that end up being too large for young snakes, or for WC individuals who are not used to a cage.

Another approach to this, as was stated, is to use greenery. This is what I did for my BP when she was small and living in a 20L (it was all I had at the time) and it worked like a charm. She now refuses to live in anything but a tank with greenery in it.

When using a glass tank, security is important. Make sure to surround the back and both sides with an opaque background in addition to providing hides and greenery.