> Another option possibly is one of those heated wires used by
> modellers and polystyrene cutters

Bit wobbly by hand, but I guess you could secure the hot wire and
move the box against a fence. I have one to cut expanded polystyrene

http://home.clear.net.nz/pages/joecolquitt/0polycut.html

and it works great on that, but most other plastics, such as acrylic
and PVC, melt without receeding. IOW, the melted plastic fills the
hole or cut you just made. This is also a problem when routing. If
you get greedy and try to make too deep a cut, the swarf isn't ejected
and the bit heats up causing melting

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