it really depends on how you want it. you should use river rock, or rock just big enough that they have no chance in eating. a plexi glass devider secured by silicon will work. but make sure it is water tight. the second set up i did leaked. it not fun to clean out water logged dirt then clean every thing again.
if you plan to do a land set up with top soil like i did in my picture, i would highly recommend a false bottom (where water will drip from the oil into the resivour beneith the soil). which i didnt do. the next time i have to clean that section out for any reason i will change it to a flase bottom.