HI All! I am almost complete on installing my electrical circuits for the basement. Anyway, I used the plastic boxes. I am used to metal boxes more, but the plastic was so cheap I went with them. Anyway, they have a plastic opening that I thought you just punch out the one side so it would hold the wire. this is not the case, I'd punch out one side, and while feeding the 12/2 wire in the other side would break, so there is nothing to hold the wire in place now. Is there some type of fitting to hol the wire? I am not drywalling yet, but I need it to hold.

Thanx