How big is the curve radius? (ok I know it's 6' from the PM)
You're talking 7/32" gap at the widest spot so there's a couple of options depending on what you have available and how much effort you want to put into it.
The easiest thing to do is take some Butyl putty tape like used for sealing RV doors, windows, roofs, etc and layer it several layers deep top and bottom and a couple of rings around the perimeter of the niche and bolt it into the wall. You'll have to trim off the excess that squeezes out but it'll seal it up so it won't leak. You'll be able to see a thin strip of the putty in the gap but it won't be very noticable.
The other way to do it is to have a machine shop with a computer milling machine, mill a piece of plastic to fit as a transition between the wall and the niche. To me that wouldn't look any better than the putty and would be a whole lot more expensive unless you know some who would it free for you.