My vague recollection is also ¼ X ¾ ,but I'm not quite sure either -
suggest you just measure the depth by inserting a thin wire with a
short 90° bend on the end, hook the inside surface, mark the outside,
remove and measure. The rivit, of course, need to be about ¼ longer for
swaging. For diameter try inserting a ¼ bolt (or whatever works) and
see if you get a nice, fairly snug fit.