@petitgugus, thank you for your reply. I have been reading your journal with interest as you take on the very difficult challenge of memorizing pi. Your palaces are very extensive and I applaud your attempt to break the national record in France. Bonne chance à toi!
I could have provided outdoor options for every choice but I wanted to keep it simple easy to compare. And I think the author of Rhetorica ad Herennium would agree with you that loci are spaces as the Latin translation of locus is background and not location used by the translator in the Loeb Classical Library. I personally use terrain so it fits my acronym of SEA-IT for types of images.
I noticed that you ran into a difficulty which was covered in the Rhetorica: “… backgrounds differing in form and nature must be secured, so that, thus distinguished, they may be clearly visible…” But I think because we have not been disciplined enough to follow and teach these early principles, we’ve lost some great advice. I want to bring some clarity as a teacher if I can.
But first, I try to follow the advice of Solomon ibn Gabirol who said " In seeking wisdom, the first step is silence; the second, listening; the third, remembering; the fourth, practicing; the fifth, teaching others. ”