The best thing I can suggest is use a ho tunnel front, though Noch has started doing one in Hoe which is 009/Ho30 Narrow gauge. this link will show you what it is like ww.noch.com/en/noch-model-landscaping-new-items/new-items-2017/tunnel-portal-single-track-for-narrow-gauge-railways-58026.html
Or have a look at this a friend of mine did of the tunnel on the Skarleoy railway.
https://srmg.org.uk/news-blog/2017/4/3/srmg-modelling-competition-2017

hope this is some help. mike