Dumb question about portal fizzler
Normally you'd have a box dropper on the level and spawn a new one if the old one is destroyed. Unfortunately there's no way I know of to detect if the player is carrying a box and trigger a barrier if they're carrying it.
Just because that is not how the fizzler works in the game does not mean that you have to cut it out. I know it is possible because I have seen it done in another map in this form. Check out some of the maps from Megadude.
Got no idea how to stop the box, though. Maybe some tool texture?
EDIT: ah, I might have something:
-give your box prop a name like "box"
-create a filter_activator_name brush entity, name it something like "boxclip"
-set the filter_activator_name's filtername property to your box' name, and set its mode to "disallow"
-create a func_clip_vphysics
-set the func_clip_vphysic's filter name to the name of the filter_activator_name.
In theory, this should work. I've not tested this, but ran into it in the VDC.