You can get any and all panels after the fact.
AFAIK all the current models are screwed together so adding it is pretty easy.
My microcenter has them in stock (some models ) but idk where you live. So do you have a local microcenter? If you were in co I could easily show you.
The big cost saving is in a) combined shopping and b) panel options when getting the case ( ie the top panel 35mm/120mm have large cost saving when bought with a case vs solo )