Consider how your bumper is mounted too. If it is installed with a piece of all-thread and expanding rubber, then adding the hopper-stopper to the bumper is almost a sure way to have your bumper knocked off (or at least draggin on one side) for a sure DNF.
The Hopper Stoppers that we sell clamp to the frame so that they don't inhibit flex or as likely to cause frame damage in the event of a hard crash. As cheap as they are, I wouldn't even bother making one up myself...and with the clamp on style, you can move it from one kart to the next when you get another kart. Just a thought.