Something like this.
Like I did on my layout.
Oh yes, I will still get there

I'm also going to paint the interior of the mountain black and add one or two LEDS at the tunnel entrance and exit.
Everything is still open, so it is still easy to get to.
I might even attempt that on the show layout. Will be much more difficult as everything is finished and closed, but I'm up for the challenge

Cheers.