1.What you need is, one empty toilet paper roll. (no paper, just the cardboard) You can always just get a tube from a FedEx or something, feel free to cut it so It will be 'just' right inside the vivarium. I'm going to have to upgrade from a toilet paper roll soon because my snake is getting so darn big!
2.String and some tape.
Get two pieces of string, Tie each one at each end of the paper roll.
After this is done, make sure both sides are even so the snake wont fall out.
3.Tie both strings together and put them through the top of the viv, if the string is thick enough and you can make a big enough knot to stay on top, then that's fine, but I would also just take it on top.
4. Feel free to add on whatever you want to it, paint, (that is safe). My snake likes to float in the air for some odd and random reason, fine with me, I can see her more!
Good Luck! I hope you find something that you'll like!