DIY tank decor/backgrounds/hardscapes

So the superglue on my sandfall (underwater waterfall) has starting failing, rocks coming dislodged. Has anyone sculpted foam and coated it to put in their aquarium? It’s in my shrimp tank so I don’t want it to affect water parameters (ph, gh, kh), but I’ll need it heavy enough to sink the foam I sculpt it out of. Any suggestions from firsthand experience?

Photo of the tank please, so we can see what you are trying to achieve

The rock tower in the center is actually the sandfall. It’s made out of landscape rocks and pvc. Held together with superglue and silicone. It’s held well the last 3-4 months, just now the rocks are starting to fall off. In in trying to reattach more fall off. So I’d like to make something more sturdy.

This is kind of what I have in mind, I just don’t know what the materials are.