When I had my cornsnake (had him about 4 years), the only time he escaped was when I was transferring him from one tank to another and had him in a makeshift tank with a not very secure lid for a day while I finished rigging the lid to his more permanent enclosure. I then spent a week looking for him to put into the more permanent enclosure (he was under my bed) and didn't tell my roommate (who didn't mind enclosed snakes, but wasn't sure about snakes being out where people could touch them).
Anyway. I had a 29gal (12x36") tank that I couldn't find a pre-made screen top for, so what I did was used wood from the local hardware store (L-shaped) to sit on the outside edge of the tank (cut to lengths, glued together to form a rectangle), stapled fine mesh screen to the inside of that. Got a thin/flat strip of wood that would fit on the inside lip of the tank (again glued into a rectangle) that sat there, to eliminate gaps that the snake would be able to access the edge of the screen. I used these clips
http://www.petco.com/product/4353/Four-Paws-Tank-Top-Screen-Cover-Security-Clips.aspx to hold it in place. In the three years he lived in that tank, he never once got out; I eventually gave him away to someone who I think is still using the same tank (and I think added a second snake), and she's never called me to say he got out of that either.
The important thing with snakes is making sure that you aren't able to lift up any edges, because they're strong and tenacious, and will go for any weakness in the lid. there can't be any gaps, and they will fit through a hole smaller than you think they can. Heavy books could work, but could be knocked down. The cage clips I had two on each long side near the corners, which seemed to work best as far as not giving me any areas of the lid that I could lift up myself. Keeping a snake from escaping is about planning, making sure everything is sturdy before adding the snake, and then being sure to close everything every time. I knew someone in undergrad who kept rattlesnakes; imagine if one of *those* got loose! ;P He had built a custom cabinet, made sure each tank had a secure lid so the snake couldn't escape its tank, and each tank was in a completely enclosed cubbyhole with a glass-fronted door and a lock. While you shouldn't need to go to those extremes, you could do a double-door system of some type to decrease the chances of escape.
Remember that feeding snakes should happen with frozen thawed prey, and outside of their living quarters; I used a cardboard box and ordered in bulk from rodentpro.com (along with friends to make shipping less expensive). Since I was in the room with the snake while he ate, I could watch for any escape attempts. (on looking at your other thread... feeding mice is required because snakes require whole prey, and are obligate carnivores... they should be just as glad that you're not wanting a full-sized burmese python, which beyond needing more room than most people can give them, are also big enough that they need something the size of a rabbit... The fur is needed to help with the digestive system, clean it out, keep it moving; the meat is obviously needed for protein and nutrition; the organs and gut contents has some nutritional value; the bones are for calcium).
