ideas for cut stone portals

Woodland Scenics makes hydrocal cast portals in both single and double track, and Chooch Enetrprises makes cast resin timber portal. Both are pretty reasonable in price.
I don't know if they are still in business, but a company called AIM made some nice tunnel portals also in N scale.

11:20PM:Just checked Walther's website. They still sell the AIM. So I guess they're still in business.