Try bending strips of plexi over heat/flame to create curvature of separation rather than straight pieces.
Make sure not too burn plexi indoor as fumes are very toxic. Just apply enough heat to bend them to shapes you desire.
If you can empty the tank and dry, use silicone to secure to bottom and back/front. The visible ends of strips on the front glass can be camouflaged by using black silicone.
As far as top of plexi, you can apply silicone and stick black gravels on top along the strips. This can be done before siliconing strips to tank.
No matter how well you separate them, some fishes may stirred up substrates over the partitions but can be minimized by gravel siliconed on top of the strips.
Have fun.