I got the idea from Shinto’s shimenawa, sacred ropes tied or attached to sacred places and trees where a kami resides. The one I used is a bit crude, but I’m testing to see what chances it has of surviving: since it’s not a legally protected shrine, it risks being destroyed or removed by the city’s gardeners. If that happens, it can be easily replaced, but a fancier rope will probably be a waste.
The altar can be found at a corner of the upper end of Eduard VII Park, right in the heart of Lisbon. It’s next to a pathway and with a small tree by its side, so it should be easy to find. The yellow dot in the following map pinpoints the location: