The apostrophe is feet, right? That’s shallower than I thought.

A single moment of madness.

I’ve done this with an old pair and works fine. It may be worth noting that I have had a few of these adapter’s fail over time. So if OP takes this solution, I would not recommend permanently fixating the adapter to the headphones (e.g. with super glue)

Out of curiosity, do you know how deep your bore ended up being?