Tree stump planter, please help?

Last summer we had a large maple cut down because it was diseased and decided to leave the stump, dig a basin in it, and use it as a planter. We started trying to saw, chisel, drill and ax the wood to create the basin but without much success because the wood is super hard. My husband does not want to use chemicals to soften the wood out of fear it will erode the stump. Does anyone have any solutions or ideas, even chemical if results are localized to the area we want to cut out? It has become a mission to do this in a timely manner. Thank you :-)