Apart from the above:
Measure bottom and use these with the depth measurement to the taper and calculate the taper? Is it an important feature?
Alternatively, use the depth to taper and bottom, then fill with a syringe to find the volume. Use that to check the taper?
Finding the diameter at the bottom of the hole could be done by test with different sized drill bits, as could the depth where the 11mm diameter reduces.
Bore it out and sleeve it, with a taper that you do know?
I’m trying hard to imagine a use for such a hole, but failing miserably. No spare imagination today.
Edited By not done it yet on 05/01/2021 17:35:32