A mine needs an entrance, but it doesn’t need to be complex.
Three pieces of 7mm square section bass wood (lime) should do it.
To add a little strength, I pinned the pieces together with some brass rod.
I had measured the sizes but before going any further it seemed sensible to check the clearances.
Good news, the rolling stock goes through!
Time to age the wood with a little grey paint and some black ink.
Job done 🙂