Fix old file cabinet: how to cut plastic box to fit in cabinet

I would recommend a high speed tool like a Dremel or Jigsaw for this with a fine blade. If you have access to a a hotknife or heated blade cutter and a well ventilated area that will work even better.

If you don't have access to these you can use tape around where you want to cut and then score several times through the tape with the exacto blade (or use a hacksaw if you have one) all the way around and ensure the least amount of stress possible is transferred to the rest of the container before cutting all the way through. It also helps if you can put each internal side up against a solid object to cut against (Like a narrow counter or table) to avoid bending the plastic while cutting it and just rotate for each side.