Reccurrent Complex has a block called "generic_solid" which can be used to create "natural floors" which help ensure that the structure looks more natural wherever it happens to generate. I think it would be a useful feature to have here as well, because often times structures spawned by your mod will be placed kind of awkwardly, especially structures with a flat bottom which have been placed on a slope. You end up with the structure overhanging the slope and looking very out of place.

https://minecraft-recurrent-complex.fandom.com/wiki/Solid