Fred,
One easy way to do it is make the wall thickness the same as the aluminum beams you are replacing. This will make them somewhat stronger than needed and about 30% lighter. You could use an inner and outer layer of 10.4 oz./yd cloth with the remainder being carbon unidirectional. 10.4 oz. cloth is very common commercially and comes out to about .014 inches thick per layer. The uni comes in various weights but you can look up thicknesses and other data on the Hexcel website. They have data sheets for most of their fabrics and uni which are pretty standard for most retailers. If you want to do some calculations, wall thickness can be further reduced while still being srtrong enough. For instance the Tornado masts I make have a wall thichness of .052 and the aluminum version is .085 thick. The carbon version was calculated to have the same strength and stiffness as the aluminum one. Carbon is 21 lbs and aluminym 38 lbs. Hope this helps.

Kevin