I'm having a problem, which I'm only noticing now that my rats are practically full grown...

I have them in this cage, which I bought off craigslist. Now, they poo wherever they want (I have bought a litter box and will probably install it tonight...hoping it's not too late to try and litter box train). When they poo on one of the upper levels, when they run around and wrestle they kick poo around and they end up around the cage and on the floor. It's pretty gross, I have to vacuum like twice a day to avoid rat poo on my floor. I've thought about buying plexiglass to go around the outside of the sides and back...does anyone else have any other suggestions?
Thanks..
Leah