I was wondering if there was a more imaginative way of doing this & thought you could cut a few fuselage cross sections from styrene & then wrap in thinner styrene to make a plug - this got me onto looking at the profile of the 787-9 plug, to find that there are two, one fore & one aft. At least they don't seem to interfere with any fairings & seem to be just a stretch of the 787-8 without any complication.
Anyway, on my "web journey" (you know how it goes), I see that BraZ Models are making a plug kit, with windows for the kit, towards the bottom of this page;
http://www.brazmodels.com/boeing.html