Here is an example of how I try to fit parts.....What you are looking at is the veticle fin from a CAREN AIRCRAFT KIT. The piece that fits around the base is a sheeted foam core that you have to cut a hole in that not only do you have to try to fit the AIRFOIL, but also the TAPER from top to bottom....VERY COMPLEX PIECE TO FIT, but as you see it is a very close fit which means I will have all the strength possible from this joint and will only need a TINY amount of filler.

I spent about a half a day on this piece alone