Here is how we make hidden belt loops so that there is less gap when wearing a tail with a belt. In the first image you can see the black spot at the top of the blue tail, that is the opening for the hidden loop.
once your tail is sewn together except for the backing, you can install the loops. I add zippers to my tails so they can be unstuffed, and this also makes it easier to create the hidden loop. Create the backing pattern as well as a pattern for the loop that fits inside of it. The loop should be cut out twice, or cut in folded fabric so you can sew it and create a tube. Make sure to mark where the tube will go on your tail backing!
I first sew the top and bottom of the backing to the tail, leaving the gaps on each side open for the tube. I then hand sew the tube into the gaps on each side of the backing. It’s hard to explain how to do this but I hope the photo helps. You need to twist it a little to sew it in.
if you want you can then cover the tube and backing with a foam piece if you want it to be more rigid. Turn it inside out through the zipper and now you have a hidden loop!