I'm glad to hear things were going well for you with regard to these eels, and I'm sorry for your loss. I think they're absolutely amazing creatures, and it"s definitely in my plan to have one in my office tank when I eventually get it started.
In the meantime, I have a ghost ribbon eel in my tank at home and it's the one fish everyone asks to see when they come to visit. Truly the fishy centerpiece of my reef tank. My tank is an AGA reef-ready tank and, to protect the eel from the overflow, I superglued a piece of 1/8" square acrylic rod around the inside of the overflow, just above the teeth, to create a lip (had to use a heat gun to bend it to shape). Then I cut a piece of 1/4" thick black acrylic to shape to sit on that lip. That keeps my eel out of the overflow. I used window screen frames and clear mesh screen to create a lid and that, combined with a completely enclosed canopy, keeps the eel in my tank.
If the spacing of your overflow teeth is wider than mine, then I could see how your eel might slip thru. On an old tank, I used black cross stitch backing from the local craft store (it's just a black plastic grid) and glued it inside the overflow teeth to keep fish and inverts from going thru the teeth and into the overflow. You have to be careful as this can restrict the flow thru the teeth, but I never had any problem with it messing up my flow and it worked flawlessly. If you combine that with an overflow lid, you might have success keeping your eel safe.
Of course, while my eel is closely related to your eel, they are different species. If you want me to share my experience in getting my eel to eat well, I'm more than happy to share - but I don't want to bore you if you think they're sufficiently different that my experience isn't applicable to your narrative.