While I was in Jakarta, Indonesia I visited EKA walet Office and saw this special pack of aroma made specially for your BH entrance hole.
I was very fascinated about the product and got a bottle.
Main idea is to make the birds believe that the entrance hole is larger that it is.
By doing so they will have a better feeling to enter the hole properly. Remember they do have those echo location capabilities and they will not be careless enough to fly into the wall.
Name: Eka Entrance Hole Aroma.
Price: Not sure
Delivery: 2 weeks upon firm order.
Call Harry for more details.