Hello,
I have – besides “regular” shrimp tubes of about 1,5 cm diameter – two quarter (2 halves of a half) coconut shells in one tank. They sit on the ground, surrounded and partially covered by leaves. I also thought the “entrance” to be too wide – and put two slightly cup-shaped leaves each in front of the entrance, hanging from the “roof”. They work as “curtains”, leaving only a small hole to enter the cave. The fish like it very much, go in and out. And maybe the risk of fry leaving the cave to early might be reduced, too? I don’t know – as I said – there are also smaller tubes for breeding in the tank, I just put the coconut shells there for additional cover. But maybe this is a way to offer even coconut shells as possible cavities for breeding?