To be a bit thriftier, I decided to build my own tunnel portals of the 2 mountain tunnels. I built them out of styrene plastic and to my own custom size and specification so they fit my track and tunnels perfectly. I started out by measuring the sizes that they needed to be, then made a scale drawing on paper to show how I wanted them to look. I then traced templates which I transferred to foam board, and cut mock-ups of each tunnel portal. I used these foam mock-ups to ensure each portal fit the track correctly.
Once I verified how the portals looked with the foam mock-ups, I used the same traced templates to cut the main portal shape out of styrene. To give the portals their 3-D shape, I used a scrap piece of square wood doweling to ensure each side was glued at a perfect angle.