Cutting bitumen damping sheets

[waxx]

I'm working on a speaker i want to make following the BBC principle of thin walls of the box, but dampened with bitumen. The sheets of bitumen (like those of Jantzen) are not in the exact size of my speakerbox inner dimensions. How do you apply the damping then, do you cut the bitumen (and how do you cut those) to size, or do you only damp a part of the panel (and the rest will also be damped)?

The box i'm planing will have an inner frame of beams of 18x18mm pinewood (giving the bracing), and the panels will be 12mm birch plywood. I want to damp the plywood with Bitumen.

 

[JonSnell Electronic]

You could apply vehicle underseal with a brush, that has similar properties but doesn't go hard like bitumen with time.
I would cut the sheets with a sharp craft knife, with care!