80 inches long, IMO, is fine for a monitor up to about 2 feet long. After that, then yes a larger one is needed. Can you tell me how wide the enclosure is? I would get the two 36 inch fixtures and span the back of the enclosure with it, assuming the enclosure is about 2 feet wide or so.
Just gonna leave this here.I agree 80 inch long enclosure isnt too small for a savannah.
Home made enclosures for large adults are more ideal, but not always possible.