You can use many things for this. I have lots of those sheets of foam insulation that I use to make insulated shipping boxes. I've taken waste strips of that stuff which wasn't going to be used for anything else, and hot glued them across the bottom of KK's. It's really easy to cut to the desired size, and works good for me.
I never use KKs, but I'm pretty sure it'll be the same thing as setting a 10 or 5 gallon on it's side. Just take a piece of plexi-glass and cut it to size, then hot glue it into place. Very simple procedure. Basically what GoTerps said.
We use plexiglass, just cut to size and hot glue in place. Then you can also add corkbard glued to the back for climbing surfaces, put a piece diagonally for a hide, make a ledge for a watering station. It works wonderfully and my arboreals love it.
I've used plexiglass and cut it down to fit on a bansaw and then used aquarium silicone to seal it in place. Of course that was at work, but they long since gotten rid of the saw so I went to a different option of useing corragated plastic sheets (from a craft store). Again I cut it down to fit and silicone it in place... it's not as "strong" as plexi, but really it doesn't need to be a rock hard solid barrier so that's not much of an issue. The only drawback is cutting it so as to not have the holes from the corragation pointing upwards (vertical) ... you want the holes running horizontal.