At least some of the aftermarket windows from Squadron are kit-specific, designed to fit Hasegawa's offerings and another for Academy's.
If you're doing the B-17G, this link is for the Academy kit.
http://www.squadron.com/1-72-Squadron-Canopies-B-17G-Flying-Fortress-Canop-p/sq9176.htm
This link is for B-17E/F, which is also probably for the Academy kit, although it doesn't say so; but Hasegawa doesn't make an "E" so it's likely for Academy. It doesn't appear that this is actually the framed "E" nose piece, though.
http://www.squadron.com/1-72-Squadron-Canopies-B-17E-F-Flying-Fortress-Can-p/sq9175.htm
They do work well. I replace the kit nose piece at least on any Fortress I do. I'm not sure which set contains them, but Squadron also provides the early three-piece canted waist windows to replace the B-17G's single piece window.
You can see inside pretty well.
