Searched with google and found some multiMatte gizmo for nuke. Tryed everything But those tools are not working perfectly. So i decided to recreate the tool myself. Here is the multiMatte v1.0.

This gizmo allowing you to create matte from 9 channels. r,g,b,a,c,m,y, luma and black.

matteColor drop down menu allowing you choose the channels. Matte erode fine will expand and shrink the output matte (used some cool tech for erode the matte)

un-check bbox when input don't have bounding box. Otherwise matte erode will introduce bounding box and expand it. 

Expanding mask control tab allow user to invert the mask channel and blur the mask input.

By-default this gizmo output only converted alpha channel and remove other channels for efficiency. User can change the output method to 'alpha to rgba'.

Values are clamped So no worries about negative values Which i found other tools.

example video