The reason it's gone wrong is accuracy - or rather lack of.
It's designed with 16 coffins, so each of them, if the sides were extended to the centre of the ring would end with a point of 22.5 degrees. Your coffins probably have 21.5 or 22 degrees.
The way to correct it is to shave off a minute wedge from each side - about two threads at the wide end tapering to nothing, or you might be able to just wrap the fabric around the papers again but with slightly less tension on the wide end.
Before you do that though, are you sure that you couldn't just join the 16 pieces together and steam press the life out of it (pun not intended!)?