drain problem

so i removed the toilet and old tile in order to install new ceramic tile flooring.  the old closet flange had rusted thru and was about an inch above the height of the new flooring.  so i removed the old cast iron flange and cut the pipe even with the new floor.  my problem is that i'd like to use a twist-n-set pvc replacement flange instead of a cast iron flange. however, the hole in my slab for the drian pipe to come thru is about 7" diameter and the drain pipe is 4" diameter, so there is a void between the outside wall of the pipe and the inside wall of the slab and i cant secure the flange to tghe floor with crews or bolts. what do i need to do in order to secure it to the floor to keep the toilet from wobbling under the weight of the user?

im thinking that my only option is to use a leaded cast iron flange or fill the void with cement/mortar and glue the flange to it.