No problem ;)

Other method (same above for the top top piece)



Trim top box by the Round rectangle for have a round face
Kill the rectangle round curve

Bridge ( only by Selection of 2 faces ) SBridge by MaxSmirnov
http://moi3d.com/forum/index.php?webtag=MOI&msg=6674.254

Select all Boolean Union = it's a solid!

Normally Bridge kills automatically the bottom face !
when you Bridge a face from a solid volume to an "alone face" (not from another volume)
or a face from another subdivised surface! (not the case here ! )
So no need to Bolean Union !
But here it's a trimed face so boolean Union is needing!

Useful when you make subdivison!
et voila ;)

Not subdivision here just Shell...(it's a solid)