I modified a spade bit to the inside dimension of a thimble. Some things not mentioned in the below links, I also cut off the shanks, to make a bit that is about 3 inches long, less flex, less chatter. Another is the end is slightly rounded like a spoon bit. Last, I drill an 1/8" hole to the depth I want to drill, this hole acts as a guide for the spoon bit, it also lets you know when you have reached your depth.
Scroll down on this one to see a modified spade bit
https://pipesmagazine.com/forums/threads/pipe-making-drill-bits.6114/
These 2 videos show methods to modify spade bits, watch them in high speed to get ideas, then regular to refine your "answer: to making a thimble bit.