Replacing Hood - Measuring Question

[Kaitie09]

I have a 20 gallon Marineland LED setup. I want to start over and turn it into a fully planted tank. Currently looking at the Versa top hoods but had a question about the measurements. The tank itself is 24", and they do sell a 24" hood. however, the top has a drop in where the LED hood goes, so the actual dimensions where the hood would drop in would be 23". I have never dealt with a new top, so I was wondering if it will just sit on top of the whole thing (24"), or if it is supposed to drop down. I'm thinking it just sits on top, but just wanted to be sure. 

 

[Ch4rlie]

A pic or links would be helpful to get full idea of what may be the case here.