That should be a very interesting DIY project. I personally prefer the colored water approach, but for those with color vision problems or a desire for more precise numbers this would be ideal. I'm assuming the grommets are a donut with a slit around the outside to grip the "sheet metal" they go thru, and an undersized hole in the middle to grip the probe? That would be much cheaper and simpler than the Heyco devices, so it sounds great. You can make 500 of these devices now and make a million bucks on ebay!