The bottom of my basket is the same, but I'm not finding it to be a problem. The walls of the basket are 9" h X 14"diam. and attached to the grate with stainless steel zip ties, and I've got a 16" pizza pan to catch ash at the end of 3 2" bolts for feet.
Are you finding your current basket too heavy or just awkward to lift in and out of the cooker?
I think a grate for an 18" would be OK, but a grate for a smoky joe might be a bit small and you wouldn't get the volume of fuel for long cooks.
One thing I've seen some of the Brethren do is have different smaller sizes of inner basket ring made of expanded steel that they drop into the larger basket to contain a smaller amount of charcoal.