3/8" HDPE, milk jug, chopping up an old plastic tank, drilling holes, then plastic welding on the cans on opposite sides. Trying to keep it pretty to the eyes, but going for functionality first.
Has to fit in the aquarium before I put it on the stand, another tank on top will leave a 4" opening.
Some people like to use terra cotta strawberry planting jars. I created a PVC pyramid early on but I never use it. I found it accumulated debris and although easy to remove/clean it dropped the mess in the water before I could get it out.
I stick with rocks now. Weight is not an issue for tank, stand or floor. The water you displace weighs a lot too.
I will have male and female cohabitation. I want an extensive maze habitat, too help curb aggression.
The amount of rock would significantly reduce the volume of the aquarium.
Weight is not an issue. Other than I need to be able to set it up with 4" of top clearance. Or put everything in it before I put it on the stand, as another tank will be above it.