Putty

Regardless of if your grip is a traditional wood one, 3D-printed, or something else, there isn't a much better way to build it up than start putting on some putty.  Some people use wood filler, others something epoxy based.  I've even seen a few with Bondo.  There are lots of resources on-line for puttying up grips, so I'll not duplicate that here.  Just know that your 3D-printed grip, be it PLA or wood composite, can take any sort of putty, just like a wood grip.  Whether you want to just put a little here or there, or make a big change like Jake did by adding some additional real estate to his Pardini's palm shelf, don't be afraid to give it a try.