at 1200 wide and 1800 high please steer away from using any thickness of float glass , if i was to use glass i would be using either a toughened safety glass or at least laminated glass (personally id be going perspex for any enclosure but that personal opinions
i could ramble and ramble on about the reasons why , but in the end it all comes down to harm minimisation , we do that(well most of us ) in every other aspect of our keeping and i think it should be passed over to DIY projects
float glass is rather weak when its edges are exposed , in the sliding glass track systems that people use it leaves these edges exposed (float is designed for house windows where its edges are held firm and tight reducing the risk of a breakage 10 fold
in the event anything was to ever hit this float glass hard enough it could very well shatter and if the force comes from the outside , it will shatter into the enclosure leaving your pet to move around in a pile of broken , sharp glass
with laminated glass this will not happen , the laminate should hold its shape and whilst you will have a broken piece of glass there should be minimal pieces come off it , toughened will break into lots of little pieces but will take alot of force to do so
now perspex , its incredibly hard to break when used in the same situations , you can drill it and cut it yourself , but some will say that it scratches up really bad , i still have pieces in use after over a year and they still look new , guess that comes down to how you treat it
oh and i was a glazier , not for all that long but this is my opinions of glass and how it should be used