One possibility for the first file
Blend don't works because it's Circle / rectangle : Circle must be trimed!

Another solution for the first file
Kill base face cylinder
Kill face top box
Select edges top box: join them
Select circle base Cylinder
Loft


Another solution for the first file

With Network but you must cut the base edge cylinder in 4 parts

Kill base face cylinder
Kill face top box
Turn 45° Cylinder
Draw 4 lines edges from top box angle to circle base cylinder (Array Circular *4 )
Cut the base cylinder in 4 parts by these 4 lines
Network one by one (or just make one and Array Circular *4)