There is a little story:
This is the place for plastic "caves"

I've bought a PVC flat ventilation channel and cut it into the pieces

[IMG:799:578]http://i12.photobucket.com/albums/a201/zzyx/cut2.jpg[/img]
I must completely destroy one side of my tank to remove stones and plants

and put plastic elements

plastic elements were accepted by the catfish very soon (there is a young Platydoras costatus inside the "cave") and they are visible from one side of the tank only

because I covered it with stones and plants, trying to restore the original layout

It works for 2 years without problems.
all the best
iwona