Can you make a cube, use a cylinder as a void on the cube. Then, take that cube with a hole in it and us it as a void to round off the corners?
I've been trying different versions of that, but am running up against tinkercad's limitations... it doesn't do true circles, and when I make interior sphere voids it's coming out flat instead of rounded. I could try to make the edges of the cube with cylinders, but then I'm still stuck with trying to round off the corners. A sphere where I "void out" the sides might work but trying to get the measurements correct would be a pain. (Part of the limitations of tinkercad is that the only measurements you can see are the full external dimensions).