Extension Box
 

The polaris copper to aluminum connector is a little bulky, I think I need an extension box, to make the volume bigger. I am using a cooktop and there is space behind the cabinet,
extending the box is OK right?

Are the extenders metal or plastic? I have not seen one, does it attach with 4 holes to the corner of a square cover box?

4" extension boxes are available and would add the the allowed volume. But these are attached with two screws, not four to the box. you can also use extension (P-Rings) rings which will provide the screw positions for the cover plate.

4" is too much extension, another 1" or so might be good.

I was concerned about how it attaches, because the builder simply cut out plastic on top to let the 8 gauge cable come from the top, I suppose the cable is too thick.

it only have provision nows to attach a square cover with 4 screws.

4" is the size of the box, not it's depth. Extension boxes are 1 1/2" deep. P-Rings are flat to 2" deep with 1/8" increments

Must it be mounted flush? If not, you can get a deeper box and surface mount it behind the cook top or range. Would be much easier and compliant.

Why are you pigtailing copper to the aluminum wires?

Is this a new install with new wires?

Old house, and appliance comes with copper wires
I have had a long question here before and the polaris connectors are approved to join them safely.