Ive never seen a nut that large with a hole that small in it. The nut is 22mm with a 5mm hole. This size nut usually takes about a 10mm bolt, plus it has that little slope on it. Where did you find this??

Another method that works just fine and doesn't cost $35-40 for the tool: Use a 5/16" flat head machine screw on the inside of the shell where you need to flare the grommet. On the badge side use a sturdy flat head washer and nut and crank it until the grommet starts to flare. This won't get you all the way there, so then, remove the screw, place the outside of the grommet (badge side) on a smooth anvil, and place either an appropriately sized ball bearing or round end of a ball peen hammer on the inside and hit it with another heavy piece of metal (avoid a second hammer, try to use something not as hardened. I use a splitting wedge. Striking two tool hardened items together can fracture or shatter.)

You can find them, but they are hard to find. The last tool I made was made from a 5/16" countersunk head allen bolt using two washers, a nut, and a plastic grommet that fit into the brass grommet. Works well, and if you go slow and swap the bolt around the other way after the initial "flare" has been made to finish the process, it usually won't split the grommet.

I've tried the ball peen hammer method and it split the grommet