Hey, I just waxed eloquent. I should have simply said that your idea to use a conventional spar will be way more than adequate. It will be very easy to execute. And it's proven.

I've seen and done a "T" spar and considered it the most elegant solution. Simply run your wing spanwise through a saw along the spar line. Glue the pieces back together sandwiching a strip of wood. Orient the grain of the wood vertically. Sand the wood to match the foam. Lay a strip of glass reinforcing tape to cap the T of the spar. You can layer the strip center span. The foam will make handling the tape easy. The epoxy will turn it into a very nice, sufficiently wide and rigid spar.