Painting with collision box

Hello, I want to make a 2D game where you are suppose to paint. But the paths you paint got to have a collision box/sphere. I have tried to use the "Instantiate" command but it's way too demanding, everytime I try to run the program it crashes unity/the built game. Is there any other way to come around this problem? Iv'e been thinking about using setPixels but i dont know how to get a collision box on the pixels.

Thanks for any help! :)