Bonding PETG to glass
The solvents that can dissolve PET are pretty nasty -- I wouldn't personally handle any of them outside a lab fume hood. If you have that, a 50/50 mix of MEK and methylene chloride should work. (Increase MEK ratio if you want faster adhesion / less working time, and vice versa.)

First thing I would try is printing directly onto the (super clean) glass. Print the first layer at high temp to try to get a good bond. Then over-extrude to get watertight perimeters. It MIGHT be better to print on a cold bed to keep the PET from popping off when the glass cools, but you would need to do some experimenting. 

You could also try heating the slide to the melting temp of PET on a hot plate and then attaching the printed part. 

Failing that, a transparent superglue could be a good approach.