The surfaces are open, so it will never be a solid.
You have to trim the surfaces if you want to have some kind of thikness in every surface, or close all the surfaces to have a big solid. Now, depends of what you want... but for sure... it will give some time to become a solid.
....the lack of "air tightness" but I can't figure how to solve that.
This one would probably take me several days to complete as there are a lot of open seams and overlapping seams. Also, there are some single side surfaces (webbing) that would be isolated separately and Thickened.
I am more familiar with the older Construction Environment rather than the Repair Environment, but in one of those environments is where I would tackle this surface-by-surface until I had a "water-tight" envelope. Not really a job for a beginner.
If this is a school assignment - I think I would pick another project.