The exposure on this photo is off a bit - the inside is actually jet black velvet with spaces for rings, chains, etc. It appears to have never been used.
Do another trial fit to make sure your mounted stitching still fits into the box lid. Turn it over, liberally apply glue and weigh it down with books overnight to make sure it all sticks securely. The glue will hold the cardboard as well as your mounting threads.
As the last step, I glued a gold braid around the edge of the block to cover the small gap between the box and the stitching - it covers a multitude of sins. Here it is in all it's finished glory!