I'm not sure I would want anything to cut a hole in the casing of the Fluke 87V. If you need to amplify sound of the piezo and add lights to it I would simply attach adevice to the Fluke so it amplified any sound it made, and flashed the lights you mention. There should be no electrical connection between the devices.
If you want the sound of the Fluke beep alone to be amplified, you can add a pass filter for the specific frequency, but if you make the box connection sufficiently close and stuff it with insulation except for where the mic touches the Fluke, it wouldn't be necessary.
Before you even build this, check if the piezo element inside the Fluke is actually working properly; just put another one is parallel, and if its louder, then maybe the OEm one is defective. Piezo elements are like capacitors, and 2 in parallel would be like addiing capacitance, so if one dominated, the other has too high an impedance. What should happen if they are ~ equal, is the sound should be reduced somewhat.
For occasional use, get a an FM mic bug kit, about $10.
Put it in a box so the mic is above the piezo element. Use an FM radio to play back the beep. Now you have unlimited volume, wide distribution, your Fluke is not modified, and the unit should be electrically safe to use.
For flashing LED, put a flashing LED parallel to the mic amp transistor on the PCB of this, as in this schematic. It should momentarily flash whenever the mic transmits any sound.
Or for permanent dedicated use, get a pocket amp like this and add a microphone to the line input:
http://www.radioshack.com/product/index.jsp?productId=2104056Then add a speaker or a piezo crystal in lieu of the headset output.
Build the LED circuit above to make the flashing lights as sound occurs.