So, I have been discussing with Kris and we do not believe there is going to be a way to tighten up the backplate with just heat shaping alone 
We think the only option may be a few small darts
So starting where it starts to lift you are going to want to cut a small V out of the plastic that goes as far in as the plate is lifted from your back. Go down a couple of cm and repeat the process until you reach the place where the plate is no longer lifting. Each V should be quite small. If you do a few large ones it will give a very sharp bend that still doesn't fit the back, lots of smaller ones create a nice shallow curve that is easy to blend in. Small amount of fibreglass and resin on the back and car body filler on the front and lots of sanding.
So something like this?
At this current time, I don't really think I can invest this much effort into darting the back panel, fibreglass etc. It just isn't worth my time when I have so many other things to catch up with (mainly studies). I could try and gently roll the arm and shoulder areas more maybe?
