Hello there. Sorry it took a bit for me to get to this, I've been out all day. Yes I did put up dividers to try to prevent some of the mixing. I used non toxic hot glue, fishing line, and non toxic crafting mesh (also called plastic canvas in some areas).
Those went into the corners. Technically I could have made one more to put in the back. In any case, I made two of these and stuck them in the back corners, one on each side. Then I put the gravel like substrate into it...and then some outside of it to start to bank it up.
After that I poured in the sand and also banked it up tight against the divider. I made sure that the sand was level with the level of the gravel, and also put smooth stones where the border between sand and gravel is and pushed those in.
So far everything is holding up nicely. The only real mixing is in the back middle so I know the mesh basket idea works because it's not happening as much with the corners.
Hope that helps if you want to try something like what I did.