I made these caves quite awhile ago and our female Betta's love'em.
They blend in so well when you use your regular substrate.
Their new tankmate, a Flying Fox, has readily taken up residence and actually shares with the Betta's... ]
We'll see how long that last when he matures.
The cave was made from 3" PVC
, Aquarium silicone, tank substrate and has three (3) openings.