Hi, i don't know if this will work or not but when i was out looking for the plastic stuff to make sliding glass doors (saw it in an aquarium shop in a tank with doors, the bottom had one slot for the glass while the top had two slots for two pieces of glass to fit and slide back and forth) I never did find any of that stuff, BUT i did fine L shaped plastic moulding. I imagine you could cut the pieces to fit the top of the tank and siliocne them together in the appropriate shape. Or you could heat up the spot where the corner of the tank is, and bend the plastic. It was at the local hardware/lumber store but i'm sure you could get it at Home Depot. I've never tried either suggestion though. I have the same tank, only a glass top and a screen top came with it. I had my crested gecko in it for the longest time until he discovered.....top comes off when food goes in, i can try to escape LOL. He's now in a larger tank witha screen top with a door.
Hope that helps 
Red 
-----
Crested Gecko Zeek:1.0
LEOS: Boo: 1.0 normal , Bronx & Nala: 1.1 blizzard,
Lily: 0.1 patternless, Abby: 0.1 albino, Zoe: 0.1 reduced pattern, Dot: 0.1 hypo
Chip: 1.0 papillion (small dog)