UV adhesive was my thought also. It is used in the assembly of medical disposables also. Once you position it and get rid of the bubbles, you would zap it with UV light. Loctite makes a number of them. I'd take a look at the Edmund Optics catalog as see what sort of adhesives they offer for optical bonding.
The other thing that might be worth considering is coating the glass with hardened gelatin as David suggests. The wet and submerge both the glass and the negative. Once the gelatin is nicely swelled and all bubbles are worked out, lift the assembly out of the water then press the two together. I would think that this will be easier than any adhesive. Keeping things neat and clean doesn't go well with handling adhesives.