On one of my previous tank, I used a piece of old aquarium glass TOP and silicone it to cover the holes.
Alternatively, you can go to Lowes, have them cut you 3-4 pieces of thin glass and sandwich them together.
edit:
https://www.petsmart.com/fish/heating-and-lighting/hoods-and-glass-canopies/top-fin-hinged-aquarium-glass-top-5230425.html?gclsrc=aw.ds&gclid=EAIaIQobChMIuKnXwoWD9wIVgYrICh1tEweUEAQYBCABEgJs-vD_BwE
Get something like this and pull the glass out and use it as it or cut it to size.