Humor speeches are difficult, especially if this is the expectation. If the assignment is to speak about the best invention ever, and "humor" is simply something which you wish to include, my advice is not to try to focus on being funny. Your topic (Duct tape), by itself, has a high potential for humor. Keep in mind that subtle (or dry) humor is often funnier than someone actually attempting to be funny.
I think there are two possibilities for the introduction to this speech. The first is to tell a personal story about an experience with duct tape. To me, it is always easier to speak and write about myself than other people. (It helps that I am also very funny.)
If you do not have a personal experience with Duct tape, Google it. Plenty of people do. There are countless stories floating around the world wide web about how "duct tape saved my life." Some of these are probably serious, but many of them are more than likely pretty silly. Choose a short-personal account to retell, then get into the "meat" of your essay by going into the details of why duct tape is such a brilliant invention. I think if you actually try to make the topic serious, and be as factual as possible, if you can make it through the speech without cracking a smile, most of your peers will likely find the speech very humorous.
It sounds like you are already having fun with this, so good luck!