Often times I find fuselage halves have enough warp, that there is not a natural flush contact area completely around. If it's minor I use rubber bands, if a major fit issue I use properly sized radiator hose clamps from the auto parts store.
To avoid either rubber bands or hose clamps touching the area to be cemented and causing a mess, I use small basswood square stock placed under the clamps, on either side of the joining line and close to it.
That keeps the cemented area clear of contact with the clamps, and also places pressure downward and inward on the surfaces to be joined. I find that makes for a tighter surface fit, less open area with no need for filler at the seam, then usually only primer with sanding to prepare for paint.
Sounds sorta Rube Goldberg, but it works for me. I hate filler.
Patrick