What is the shape of the bottom now? You can find end mills with radiused corners but the problem will be holding it. Unless you have the shank relieved, a conventional extension will not fit. Ideally you could find a 3 flute drill and regrind the point to the desired shape. That would more likely be long enough and the 3 flutes would help to center the cut better than a 2 flute drill. I would not try a spade drill, as without the point, it will not center well.
“If you're not a liberal at 20, you have no heart; if you're not a conservative at 40, you have no brain.”--Winston Churchill