I know this is a common issue, but I'm having problems getting the object in the attached model to fillet. It's a sphere with a letter boolean'd out of it so that the text appears punched into the object (or at least that's the idea). I want to fillet the edges to give it a softer, more organic look.

However, either fillet gives no feedback at all and just fails, or it sits there calculating. Is this just the wrong tool for the job? And if so, what should I use?

TIA
Dan