You can't. When you want a hollow shape, you have to make it out of non-hollow shapes. Want a box with an open top? Make a bottom wall and four side walls and put them together to form a box with an open top. That's exactly what CSG Hollow does (when it works), except you have a lot less control over it.
I think one reason in many cases making a hollow shape by drawing the individual brushes instead of CSG hollow is that in the former method at the lines (/etc?) where the brushes meet, you will more likely remember to use metering , which means the brushes will not overlap and their faces will meet conveniently and the unseen surfaces will be invisible. With CSG hollow I think the brushes automatically have overlapping portions where they meet.
Make Detail disables collision detection on a brush. Use it to add detail that you don't want to interfere with gameplay.
I remember the editing recommendations said one use of detail brushes is on brushes that the ball will never touch, saving the computer a bit of processing work (wutever it is called).
Off the top of my head the only way to make detail brushes distinguishable from non-detail brushes is to click "view > filter > ('structural' or 'details')". Then all the detail or structural brushes will become invisible (you cannot edit them, probably).
(forgot to say this in last post)
One possible reason CSG hollow might not work is here. Well I think when you do csghollow the width of one of the resultant border brushes is determined by the current grid size. So if one or more dimensions of your brush is less than 2 times the current grid size, there would be no room (less than 0, so a negative number) when you try csg hollow, meaning the command might not work.
ps this thread is getting kinda long
It the it's level set: http://neverforum.com/fmpbo/viewtopic.php?id=29226/19/17: a bit of help needed here, plz: http://neverforum.com/fmpbo/viewtopic.php?id=2988
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