You won't find a long glass canopy like that .... for one, it would be too unwieldy & too easy to break.
You can build one yourself if you want. You can buy the plastic hinge, and have a glass shop cut 1/8" glass pieces to size. You just slide the glass pieces into the hinge & you have your canopy.
An Eg of a place that sells 48" hinge:
aquarium, fish tank, door, replacement hinge, clear and black.
A 48" glass top is too unwieldy, IMO
. You might be better off making a center brace. You just get the glass shop to cut you a piece of 1/4" glass 2-3" wide & exactly the distance of your aquarium depth (front to back) minus 1/16". You silicone this piece at the center of your tank. <You might have to silicone a plastic shim at the center of this brace to bring it up to the level of your existing rim.> This is called a "Euro-brace" and is the standard way to make the center brace (and rim braces) in a DIY tank.