Concrete tunnel portals on the UP in Weber and Echo Canyons have very distinct impressions of the wood forms used to pour them. Texture is more like woodgrain than anything else, but today, the portals are all cracked up and stained. Here's a prototype photo of UP double bore tunnels at Taggarts...
Photo (1) - UP Double Bore tunnel portals at Taggarts...
I used these as an example to do double bore tunnels on my buddy Nate's layout, but the track was laid too close together to get an exact match. However, I copied the weathering and cracking on the prototype pretty closely...
Photo (2) - UP Double Bore tunnel portals at Riverside on Nate's (@Nato) layout... 
These weren't 3D printed, but made out of premium pine, and I'm no expert at drawing 3D models, but these photos give a good look at well-weathered concrete tunnel portal surfaces, weathering and coloration.
As for contours...yours would improve with making the arch circular rather than a slight elongated oval, and keep the sides either parallel with each other (for low-pressure tunnel bores) or curved inward (for extreme lateral pressure tunnel bores). Here's an official UP drawing that shows tunnel bore proportions and dimensions which would be what the tunnel portal opening look like, which will assist in your 3D model by giving you actual measurements. However, I would still check the clearances with your trusty NMRA Standards Gage before installing on your layout.
Photo (3) - UP Common Standard Tunnel Clearances:
Cheerio!
Bob Gilmore
