Hello Patrik,
My advice is still forget this project. Someone is leading you astray. For example; tapering a mast from 17.5cm circumference at the hounds to 14.5cm circumference at the top does not reduce the mast stiffness 64% at the hounds. Someone is telling you homespun physics 101.
If you look at the Bethwait mast/sail, you notice the leech of the mainsail intersects the top of the mast at nearly 90 degrees because the mast is built with severe aft curve in it. This causes any increase in leech tension, due to a puff for example, to pull directly down on the top of the mast and induce more mast bend which flattens the sail and opens the leech.
On a typical catanaran rig with a relatively straight mast, the leech load from the mainsail intersects the top of the mast at about 15 degrees. (The tangent of the head angle equals the foot/luff for a straight mast.) This means that 25% of the leech load goes into bending the mast and most of the load increases the axial compressive load travelling down the mast. In contrast to this the Bethwait rig puts 100% of the leech load into bending the mast and the mast responds. With only 25% of the leech load going into bending the mast, the bend response would be much less. To build a mast with built in curvature like the Bethway mast, requires two molds, one left side and the other right side. This is a big project!!! Too big for a hobby.
Bill