I use aquarium silicone to glue two plastic rods to the glass far enough apart to let the eggcrate slide between them, then do the same on the opposite side and just slide the eggcrate down into place between them. this is very secure and it is easy to remove the sheets for cleaning, and it is not hard to remove the rods and silicone if you don't want the dividers anymore. Average size on the smaller grid egg crate is 1/2" which is good for most cichlids and medium to large fish, (it it sturdy enough to withstand a determined attack from a 14" rpb or a 36" american eel.) If you get fry or very small fish get some appropriately size plastic screening an d silicone it to the grid. That will reduce the water flow a bit but a lot less than those white plastic perforated sheets the lfs sells (which an 8" oscar can break through), again when you no longer need the screening the silicone can be removed with a plastic window frost scraper and white vinegar.
As an aside has anyone ever used white vinegar to adjust ph?