Since it is large and deep do not just fill with putty, it will shrink and crack over and over and when you paint it the moisture from the paint may make it crack again. Fill the majority of it with sprue, styrene discs that you cut from sheet or some form of plastic this will work with your solvent glue.
Once it is mostly filled use a putty that can be wiped away rather then sanded. Use perfect plastic putty or mr surfacer 500 or bondo thinned with acetone to make a paste. This way only a minimum amount of putty is needed. Depending on which putty used you can use water, acetone or lacquer thinner to wipe away excess putty once dry with cotton swabs.
Once you feel it is mostly smooth you can finally do a light wet sand with fine sandpaper like 800 grit or higher.