I've done exactly that. Carriage bolts being upside down so the heads rest on the grid below. It makes a fairly solid mount. On the diffuser plate it would probably slide around but if a guy drilled holes where the bolt heads sit maybe uh 3/8 hole /the convex bolt heads are probably half inch then they'll sit down into that hole a little bit and which should work fine.Would not be "elegant"- but you could buy a few dollars' worth of carriage bolts, nuts and washers to make 3 (or 4) legs that would sit on the outer edge of your diffuser plate. ...
The melting point of zinc, the material/element used in galvanizing metal is 787F. Once burned you could remove the residue with a wire brush. Spending the extra money on stainless bolts can avoid the zinc concern if that bothers you. I use stainless steel bolts on my Webers not so much out of the galvanized concerns but for the clean up concerns. The stainless steel bolts clean a lot easier than the galvanized bolts.
I am not sure how much higher you want to elevate your cooking grate from its usual height. When cooking burgers or bacon using the indirect method, I will take another cooking grate and turn it upside down and place it on the first grate so the handles of the two grates are 12:00, 3:00. 6:00 and 9:00. This doubles the cooking capacity and the distance between the tops of the two grates is 2.25 inches. I hope some of this information helps.
Would not be "elegant"- but you could buy a few dollars' worth of carriage bolts, nuts and washers to make 3 (or 4) legs that would sit on the outer edge of your diffuser plate.
Zinc plated would work fine (if you are one those "but what about the fumes?" folks, there are ways to mitigate that - or pop for a few more $ for stainless steel hardware.)