Hi Gordon. My apologies for this late reply: I didn't get a notification about your message. Sounds like a wonderful project. Good luck with the amplifier. I have found another, commercially avaiable one that's an alternative to the now hard-to-find Radio Shack amplified speaker. it's the Danelectro Honeytone N-10 Mini Guitar Amp. https://bit.ly/39fznCD I bet you can get it in South Africa. You should get in touch with Albert Chakona at SAIAB in Grahamstown who is pursuing various projects on mormyrids and who has some experience recording their EODs.
Good luck and stay in touch!