Hi,
I did something similar, where I had a set of floors and wanted to map something to them (bottom surfaces, in my case).
I found Jeff Shaver’s method extremely useful (details here: Floor points). You would have to go through a similar process of filtering the points generated by the top and bottom of the walls (instead of floors in his example).
If you like it, please make sure to give Jeff some kudos!
Good luck-
Joe