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.
It doesn't matter if you're still trying to find the black, or have been a High Master for so long you look at a 9 as a tragic mistake, you're probably never going to be done working on your grips.