Rubber doorstops, they come in a variety of sizes (Google tells me 20mm to 76mm height). Screw the grille into the frame as per @Phil Starr then some door stops near the middle, black washers and bolts and T nut them in. The advantage of these is that you can position them to kill any unwanted vibrations in addition to improving the kick resistance. Strangely in 55+ years of gigging this has never happened, I must be playing the wrong gigs. Plenty of impacts with doors, furniture, car parts and other kit though.