FINAL RESULT !!!
 
 
Starting out with simple plastic boxes:
Cutting 2 boxes up and gluing them together with aquarium silicone:
Checking the fit in the 160 ltr aquarium:
Modelling the cave with Makemake aquarium concrete:
Decorating with small twigs and gravel:
Close up of the decoration:
Checking the fit again:
Attaching more roots with silicone  inside the cave to simulate roots penetrating the cave-roof/layer:
And a nice result, I think:
Applying the 2./decoration layer - also creating a small wall to keep in the aquarium soil I intend to put on top:
On the overhang I intend to plant/attach mosses and opened-up algae-balls:
I saw on Youtube that you can attach moss with superglue, so why not with silicone:
Ready for action:
Here I fill the biggest voids with filter material:
..and then cover everything with Tropica aquarium soil:
A quick "dry-run" of my intended aquascape:
Here I attach the moss strips to the overhang with silicone, and start filling up with water:
Tying mosses on to roots with ultra thin and strong fishing elastic string:
And here the tank is beginning to take shape, with mosses fitted strategically:
Foreground plants, Eleocharis acicularis "mini", ready for planting:
Planting:
Hemianthus callitrichoides "Cuba" & Eleocharis acicularis "mini":
Attaching taller plants like Microsorum pteropus "narrow" to articficial, sinking piece of root:
Now is the waiting game, letting the plants grow and spread...:
The plants have grown (along with all sorts of algae), but it starts looking better. Also, I have finally put some fishes in there:
FINAL RESULT: