I like the look your going for!
If it is a bit too light maybe silicone some flat slate to the bottom sticking out past the rockwork and then using some heavy rocks on top of the slate to hold it all down? If that makes sense?
I know what you mean about a tank that size big too small even for fancies. I have a 70g that I’ve used to grow baby fancies just over the winter months and I feel it’s so cramped by spring.