I did 2 similar models using different methods to understand the positive/negative part of my underground design.
Negative on positive
Positive on negative
I am currently exploring new ways to do the physical model, to show more of the construction stages of the whole underground structure.
As suggested by Ed, I could try doing different construction stages in layers from bottom to top (similar to the laser paper cut-out method I did last time) to also show the construction timeline.
Bruder Klaus field chapel
The church are made of 112 tree trunks premises. Once finished form work, concrete layers were tamped over the existing surface, each with about 50 cm thick and with 24 days of daily poured layer. When 24 layers of concrete were poured and dried, the wooden structure was burned, the slow burn lasted 3 weeks, leaving behind a hollow cavity blackened, with a negative walls charred in which there were few traces of logs original.