How to make BoxColidier to have transparent side?

[VimLoudy]

Hello everybody, this question is so hard to explain… So from this picture i think you will understand… So on the picture we have 1 field which is marked by BoxColidiers but 2D .
So when we have object inside of the field, object can’t go out from the field (For that i use BoxColidier2D). But when object coming from the outside of field, it can enter to the field Please if you understand what should i do? A picture is attached…
Thank you!![![alt text][2]][3]

[AmunTech]

You can set the bottom collider as trigger, this let the obect pass, and with a simple script you can activate it when the object pass through, something like this:

void OnTriggerEnter2D(Collider2D other) {
    if(other.tag == "ObjectTagHere"){
    GetComponent<Collider2D>().isTrigger = false;
    }
   }

or

void OnTriggerExit2D(Collider2D other) {
        if(other.tag == "ObjectTagHere"){
        GetComponent<Collider2D>().isTrigger = false;
        }
       }