One of the things that happens up here when people are sewing tough or rigid fabrics is that they can’t get a good professional press.
If you have a tailor’s point press, anvil, clapper, and some clamps you will be able to get that thing to lay flat as a flitter.
Get a lot of live steam and heat in the seam, then clap it between two pieces of wood and fasten. You can take the clamps off when it cools and move on to the next section.
It is especially helpful when you are doing belt loops or complex junctions in heavy fabric.
It just looks ridiculous.