/3/ - 3DCG

you can do this in a minute. I used a low sided cylinder to start so its blobby, you'd just up the base to get OP's pic.

make a cylinder divisible by 4
extrude a segment out and scale outwards
extrude another segment, this is end of box
select cylinder sides / 4, make planar until box
chamfer the edge loop from cylinder face to box.
done