Answering your question:
In making the topical solution of fortified ceftazidime 50mg/mL you can use either SWFI or BSS (balanced salt solution) or normal saline. Many compounding formulas you will find use either of these, most common is normal saline or BSS though (helps to be more isotonic with the eye). Attached are 2 publications for the stability you are seeking. These are for 24 hour room temp, 7 days fridge. Of note, this is also the same ceftazidime stability for the parenteral dilutions. I have included another nice reference for many ophthalmic drug dilutions to many routes, intravitreal, intracameral, intraoccular.
For others not familiar with *fortified* ophthalmic drops here is a nice description below.
These are usually used in endophalmitis (inflammation/infection of the cornea and inner eye)
Fortified preparations of ophthalmic antibiotics are made with commercially available antibiotics (parenteral or lyophilized preparations). These fortified eyedrops have two main advantages: the increase in the antibiotic concentration in the corneal stroma and the wide choice of available antibiotics. Fortified ophthalmic solutions are used in severe keratitis (large diameter, stroma infiltration, inflammation of the anterior chamber, old patient). The following associations are recommended: ticarcillin+gentamicin+vancomycin or cephazolin+amikacin since they provide a broad-spectrum antibiotic activity against the wide range of bacteria that may cause keratitis. The main toxicity of these preparations is the retardation effect of the epithelial-healing rate (aminoglycosides, vancomycin) and the corneal and conjunctival toxic effects (aminoglycosides). However, fortified antibiotic drops remain the standard therapy for severe bacterial keratitis, given their corneal penetration and the possibility of the synergic and combined effect of an antibiotic association.