They actually sell a light canopy for 2.5 gallon tanks. You probably have to order one online since I haven't seen one sold in stores for years now. If I remember correctly, the light canopy still sits on the glass canopy. You could probably order a glass canopy for a 2.5 gallon online.
Anyway, if you haven't already, try searching online. I know they used to sell canopy for the 2.5 gallon cause I had owned one. Wish I still had it.