Is it a tall critter keeper or a short one? the reason I ask is because I keep alot of my younger snakes in short kritter keepers from time to time. The small one is about the size of a shoebox, and the large one is a little larger than a 10 gallon (in floor space) Both of them also have sopts for those little lights too. If he's in the tall large, you could probably keep him in it until he hits about yearling size. If he's in the short large you could keep him in there until he hits about 2 feet. Most screen lids for 20 gallons have 2 pieces to the frame and the screen normally fits sandwiched in between the frame. Not all are like that though, so check to see if yours is different.
Check all hides and under the substrate for the shed. It may be in pieces since he didn't shed the last 4 inches or so. If you use aspen, sometimes its hard to find. Dig around pretty good and it should turn up. If not, he may have eaten it. There was another thread posted not too long ago about a snake accidently eating its shed, but it could still be a possibility.