In the wikipedia article it mentions about alum being soaked into cloth so it can be used to cover things, that makes me wonder if a piece of alum saturated cloth would work - remember it would be sandwiched between two layers of varnish. However, if I get around to doing an early period Did story it would be based on portals distributed by the system creators.
Now, there are a few stories where people find portals, but in each case, so far, it was a portal owned by an ancestor who built it into their house or a cupboard that stayed in the family. There is one story where a fellow moved a portal from the middle east to the USA.
In the early DiD stories all the rooms are plain white, and the only personalisation done is by the Caretakers, with furniture being the only item before the manuals are found in recent times. So a plain white room for a first meeting with a Hero Greeter would be correct, regardless of the period - unless the person is introduced by an existing hero.
The Mark II portals only require a ring with a suitable sized sapphire gemstone, so it would be possible for a person who is wearing such a ring, be it an old portal ring or just a sapphire ring with the right sized gem, and is of the right nature would be transported on passing through a portal. Thus they could be walking through an old building and be sent to Crossroads when they walk through the portal without knowing what it is - it could even be a ruined building. So your idea can work. You could even have a fellow obtain the ring and the house with the portal as spoils of war and he becomes a hero, or the spoils get passed a long a few generations until a new Hero wanders through the portal while wearing the ring.
The system was established in the 13th century, so you can have someone finding or inherited a ring and wandering through a portal any time after that. I just wouldn't expect any in the Americas until the 18th century, but that leaves all of Europe for the 13th to 18th centuries.
