John comments:

“Wow, great tips and very useful! Tunnels can be made with insulation foam, but buying a portal and fixing it up as you suggest is really good. In fact I have a Life-Like (anything but) small N-Scale tunnel that I was wondering what to do with, and I can dress it up as you said with paint, ground cover, trees/bushes, and a little lightweight hydrocal, as well as detailing the rocks on the tunnel openings.”