I set up my 29 specifically for my first plagiostoma. Reverse-flow UG filter run by an aquaclear 70 powerhead and an air-driven sponge filter to collect debris. I never even broke out the pieces along the back of the hood, just ran cords and airline through the small gaps for them in the hood.
The 55 I set up, intending to move them into after I got another plagiostoma, was set up the same way with the UG filter plates, and I also used a fluval 304 on that tank. I have versa-tops on that 55, and cut out the plastic splashguard just enough for the intake and return hoses to fit into, with the powerhead and heater cords running out the same cutouts.
IMO there isn't any really good way to keep a spiney eel out of a hob filter, but my best suggestion would be to tightly roll a piece of plastic canvas and figure out a way to secure it in place so that water flows through but the opening is blocked.