I use a fair bit of 5 minutes epoxy, and try to make the mating parts have some sort of keying and registration, with both help with alignment and with strength of the joint.
Then I go over the seam with a sharp knife/chisel to remove any squeezeout and blend it.

After that, it's a whole lot of using putties, filler primer and sandpaper to get it smooth and hiding all the signs of it having been 3d printed and assembled.