Here is a modification of the script provided by kingofmk:
;Artificial Intelligence Script
desc = 15 Sec Motion Sickness Trigger Zone
;Triggers
:state=0,plrwithinzone=1:setposteffect=motionsickness,state=1,etimerstart
:state=1:plrwobble=35
:state=1,etimergreater=15000:setposteffect=X*,state=2
;End of Script
Attach this script as main script to your triggerbox.
This script requires you to manually replace the marked
X* with the standard fullscreen shader you are using when the player is not
dizzy. Otherwise it won't work as intended