Make one and weld it to the bumper?? or buy one??? Seems cheaper to buy one but we have one that was made and welded on another kart (we bought that way) and it holds up great.
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.