Don't use a stringer, they don't do anything useful. Use carbon everywhere you though needed reinforcing, especially around the mast tracks. Carbon is so much stronger than glass, and the extra cost is marginal if you're wrapping the whole board in carbon. You'll have a lot of off cuts that will be useful reinforcements. If you run your finder down the mast tracks before you bag it you'll easily see them, a trim router with a straight bit that has a bearing on the bottom is the best for cutting the carbon out. Dremel's don't have an equivalent tool but you could use a hacksaw blade to cut out the bulk and then sand the edge with a block. Just try not to squeeze resin in there. You could do the inserts afterwards but what a pain, and you're better off having that nice skin or carbon without the cuts. Either make a template so you can find the holes again, or wiggle a tooth pick or something into them before you bag the board so you can see at least one of them.
The best site I've ever seen for this stuff is here (someone posted the link in an earlier thread, thanks!):
"Picture the fiberglass skin (the steel cable): it is immensely strong (approx 25,000psi) and will not stretch appreciably (elongation approx 1%).
Let our stringer (the rubber band) be made of Cedar, with a tensile strength of around 450psi (and that’s being charitable), and an elongation of approx 5%, i.e. about 5 times stretchy-er than fiberglass.
As a load is applied, the fiberglass skin will remain virtually unchanged, until finally, as its yield strength is approached, it will stretch a minute bit, and then snap. The Cedar stringer will initially not see any load whatever, because it is happily bending along, not even close to its elastic limit. By the time the fiberglass gives out at its 25,000psi tensile strength, and all the load is transferred to the Cedar stringer, it – with its puny tensile strength of 450psi – is hopelessly outclassed and will fall over dead immediately."
Say no to stringers! Sorry, I know you said your blank didn't have one, but it's a thing of mine to campaign against them.