Is it possible to make a brush that shows doesn't draw anything in it, except the skybox

[fresh karma]

I don't think so
because the skybox always renders behind the world

anything that's in the PVS will be visible

closed areaportals basically just make everything behind it no longer in the PVS

potentially visible set

the set of visleaves that is possibly visible at any given time

which means the area of the map that's currently being rendered