I used a slightly different method to build a rudder. I purchased a sheet of rigid crosslinked pvc foam, cut and sanded it into a core then covered if by wraping the glass so that the ends of the cloth came off of the trailing edge, then trimmed off the excess after the epoxy set. This saves you from building molds and is much more resistant to splitting. If you use unidirectional carbon cloth lengthwise down the rudder it will be able to use fewer layers since it will be oriented to the direction of the largest forces. My rudder was just for an old catalina 25 and it was severely over built with 3 layers of 10oz unidirectional e glass and 2 layers of 10oz biaxial stitched material. I would think that 2 layers of each would be adequit even for your application. If you do use a biaxial stitched material, get the +45 -45 oriented rather than the 0 90. I found it much easier to work with.

Good luck, keep the epoxy out of your hair.