I am sure you have seen this but in case you have not?
Second Strike Air Cleaner Calculator
By this calculator you are OK with two paper filters. It is interesting to swap in the oiled felt filter and see the difference.
I have this setup at present, when I first saw photos of it I did not like the look of it.
It grew on me over time. I am a bit of a function over form type guy.
I did the calculations in the past and the top does add quite a bit of surface area.
Good luck!