For filling cracks a couple common techniques are
- UV resin and UV flashlight
- Automotive body filler or spot putty
Then it sounds like the overall look should fit right in. It sounds like the weapon will be very 'in theme' with the costume.
I think it turned out quite well looking at the end result. (I wasn't there to see all the work to 'convince' the parts to go together).
I know resin isn't cheap. Smaller props might be a good way to dial in printer dimensional accuracy and practice gluing/seaming techniques - without breaking the bank. I personally like printing a nut and bolt as a test of accuracy: When they thread properly together you know you're pretty close. A couple small test parts just to practice assembly might save some money. For example: Print a speed shape in two halves then assemble, seam and paint to the point where you'd never know it was two part. Speed Shape for paint tests by PandaProps