Measure small voltages in a high voltage circuit
You are right in assuming that you cannot rely on the meter insulation between you (0v) and the -2000v circuit you want to measure.

You can make the measurement safely, if a little more slowly, by isolating the meter and its probes and wires from the ground. Get a suitable insulator, say a glass or plastic bowl or chopping board, and place the meter on it. This is obviously a battery powered portable meter, not a mains powered one. With the target powered off, set up the meter and connect it. Dress the probes and wires so there is an air-gap, which is resistant to the set-up being bumped or knocked, between meter probes and their wires, and any ground conductors. This is why a bowl may be a better solution than a board, you can allow the excess meter probe wire to sit in the bowl with the meter. Power on, read the meter without touching it, and power off. Wait for high voltage capacitors to lose their charge before unclipping the meter.