well there is an easyer way especially if you want ease and simplicity. A one peice mold made with clear rtv it can be had from micro mark or BJB ent.inc. has a great one as well. an expample of how it would work since you now have a clear mold study the mold carefully and determine where you want to cut it I would say right on the edge of the fins. extract the original. clean up the mold then cut up some small scrapes of 1/4 bass wood plank to act as backboard stiffener place these on each side of the mold and hold them in place with a rubber band. now just mix the resin and pour.
Now get to it.
let us know how it comes out
You could have put thin plastic card in the areas between the fins and the body.They would add natural opening points in the mould and be easily removed after casting...like the flash that you'll have on that mould.