decal with random rotation

I have almost perfected my decal system, I am trying to add random rotation and scale to the decal but I cannot get my rotation to work I have tried using the transform of the decal and rotating it using x,y,z and up forward etc, but the decal always rotates the wring way. I want the decal to simply turn randomly in it the facing plane.

any ideas on how to ensure that the rotation is only on the forward facing axis?