I don't remember, what exact opamp I used, but it was made of pure obscurium. MCP6283 is a good replacement. If you need a higher voltage amplifier (for +-15V supply), LF356 is an okay choice. The only real requirement is that the opamp should have low bias current to create no significant dc voltage across 3M resistor.

The trimpot can have any value from about 1K up to about 100 K. The lower the value - the lower the extra noise.

LTK8002 can be replaced with any similar audio amp. Examples are mix2037 (excellent, but unobtainium), NAU82011WG (unobtainium), tpa6211, lm4871

The enclosure should be seriously airtight for best results. I recommend adding vibration isolation between the mic and the case to prevent unwanted mechanical coupling to the speaker through walls.

The speaker should respond to tapping, but not oscillate too much.

Here's a video. Sorry, it's in Russian... but I've added English subtitles, use them.

And it's done!