How to transform this geometry into a solid?

Hello everyone,

I am using grasshopper for a short time and would like to know how can I make this geometry a solid by filling the space between the surfaces… In rhino is pretty simple, since i would have just to use the shell comand. But in grasshopper things seems to be a little different whenever it needs to do a brep split or simillar… The geometry is internalized because since i am new to the software the code is very messy.

Thanks in advance. (10.5 KB)

It is the same to use Boolean Difference (20.0 KB)

Hey, thanks, but i didnt understand the extrusions, could you please explain me?

thanks again man, really trying to learn gh

here I separate the contours involved. I create a bounding box and then use it to apply a boolean difference
you can use this view mode, to see each step
image (14.6 KB)