Remove to your taste. I take out just enough to barely expose the clear area all around.
I found a saw sharpening clamp was the best way to hold the two pieces together as I worked.
Patience and check as you go. I measured first and scratched guide lines where I wanted the cuts to go.
You only need to do 3 sides, because you can slide the negative left-to-right.
When done, use steel wool to remove burrs. Paint the edges with a matt black paint to reduce reflections, I used "Model Master" Flat Black enamel paint. There are probably better, flatter paints.