This is the one I ended up making which took around an hour with my Radio Shack electronic skills, lol.
$12 for amplifier (suggest EBAY)
$8 on clamps, bought a pair just in case a messed up the welds (Home Depot or Harbor Freight)
$7 to $10 on headphone cables (Radio Shack or sometimes Dollar Tree)
Explanation of engine knock and detection tools:
DIY Engine knocking detection tool
The cool thing about the electronic amplifier is it can be relocated to hear the transmission, cylinder head, crank, bov, compressor, etc. because it uses a clamp compared to bolting it to one specific location.
Note: One difference with my setup is I used a headphone cable leaving the plug side available. The clamp can be left on the engine bay and whenever I want to hear noises its just reaching to the glove compartment and plug the headphone amplifier. There is barely any static at all, sounds are crisp clear. Only problem is when it is clamped near the fan so try to use an area distant from the airflow.
I tested it on the beater stock Escort GT (1.8 dohc) and I could hear very clear engine knock at 2,000 to 3,000 rpms which I never ever detected using a 8" extension placed straight on the engine block.
It is cool because I can hear the shifting, when the clutch engages, the turbo spool up, bov activate, the sound of the valves opening and closing, ticking of injectors. At work my friends look at me as been crazy, lol, although they found very interesting how everything can be heard over the headphones.