I've done some smaller changes to the circuit, most noticeable omitting the microphone and microphone preamplifier as well as the output amplifier. I also changed the delay time range by using a 100k potentiometer instead of 22k (RV2) as well as 1k resistor instead of the 10k (R2). Since I have a +5V option installed in my modular I made some changes at the power supply input.
Unfortunately there are no building instructions for this module, but at least you'll find the schematic, some photos, and some comments.
The finished module, without a faceplate.
The finished module.
Here's a demo of the delay.
That's it! Have fun and create some echoing space!