[Big hammer]

Back up the D-nut with a bucking bar, Then with a ball peen hammer peen it over. All you want the D-nut to do is not turn or fall out. That's if you have room to swing the hammer, if not use a C clamp with a large ball bearing and again hopefully you can reach with hole. If these don't work, if you get steel D-nuts you could tack weld them. All else fails JB weld.