Infinite collider

If your motion is without physics, you can just constrain the target's X and Y axis to where you want ( you can even use Camera.ScreenToWorldPoint. It's been a while but I'm pretty sure ScreenToWorldPoint(new Vector2(0,0)) would get you the top right, and 1,1 would be the bottom left. 1,0 is top left, and 0,1 is bottom right.

Alternatively, here's another answer that actually generates the collider boxes: https://answers.unity.com/questions/1442077/top-down-shooter-boundariesspace-shooter-creating.html?childToView=1442118#answer-1442118

A quick alteration of that putting that last set of code inside of an Update function should cause it to stay live in case your player actually moves through the world. Depends on how your game works, though.

I just have a long flat floor and I want it to go on forever, thanks for replying anyway.