Fill Cavity in 3d model?

Hi, im trying print this model:
bot.stl (847.1 KB)
It has many cavities inside and I would like to fill it in and make it opaque
Could someone tell me how to do it?

What do you mean ‘fill in?’ I didn’t have any problem slicing this in Cura.

I want to split the piece in two leaving the face smooth but with rhinoceros it does strange things, im using meshbooleandifference:

this is a example that I want to do with the new stl:

First use the command _meshrepair to middle section then the split.

Can you explain to me how to do it? I also wish it had no hole inside
sorry i’m new in 3d editing

Hello - the two ‘side’ pieces are full of self-intersections - (MeshSelfIntersect command) that is likely to cause problems with many splitting or Boolean operations.