Hi Brian,
I use PETG filament in my 3d Printer as it's an excellent impact absorbing option. Glueing it though, is the pain.
Done lots of experiments and, on the filament I use, many others concur that Gorilla Super Glue is the best for getting a strong bond, others don't get anywhere near as strong a bond and I've seen lots of stuff fail due to weak PETG adhesion.
Sadly I can't comment too much on the fogging as 3D printed objects don't have the clarity of a sheet of transparent PETG but I haven't noticed any fogging.
If it's a non structural part or something you can get away with perhaps framing in someway (like a window frame from behind?) then I'd go with an epoxy solution and leave the frame to hold it in place.
If you do need to bond it to other bits of PETG then Gorilla Super Glue is by far the best available.
Hope that's of some use.
Alex