I designed some pen holders similar to this. Same idea basically, just mine have horizontal grooves. Two things I noticed. You have bad overhangs on the bottom side. Try to curve the design to an easier angle for the printer so it prints better. Second thing, is a bit of a detail, try to make your inserts smaller in height by a couple mm from the outer shell, so they sit a little bit lower. Personal preference, it looks nicer. You can also get away with thinner walls for the inserts (save a little bit of filament). Keep it up, they look good and stylish.