Asking Collective Wisdon Gnawing Plastic Reinforcing Ridges

Mex,

If you plan to run that tool in your good drillmotor, be ready to kiss it good bye....

No drillmotor has bearings for that kind of side load.
If you can use the flexshaft tool to do it, that probably has the bearings for it.

Me? I would start with a sharp wood chisel and a hammer until it was mostly gone. That way I could cut nearly flush to the bottom.

Matt