Light Cookies on imported Models

I want to have a very simple light cookie , a square cookie . I wanted to only point the light to a certain part of my 3d model , I wanted the light to only lighten a square part of my model . I could extract that part and use Culling Mask but that would make my application size bigger and make it use more process . I wanted to use a simple light cookie, the cookie works perfectly if I use the light on a simple cube , but when I use that light on my imported 3D model , the cookie doesn't work at all and it seems a normal light without any cookie . I wanted to know if light cookies does work on imported 3d models at all and if there is a better solution to my problem other than light cookies .
thanks