We have one of the tube type fish traps you can borrow. It has a trap door that slides down. I think Doug is finished catching fish with it. We used it to catch a large sailfin ( now with Joe Fitz). You should be able to catch the hippo with it. The DIY is just a plastic hinged box. You wrap an elastic band around it with the stick to keep it open. A sting is tied to the stick, when the fish enters the trap the string in pulled. Like the old bird trap with the upside down box. The problem is that you need an elastic that will close the box quickly to trap the fish. This means the box usually goes flying knocking corals everywhere.