Elad:

A good quality, well cured spruce 2x4, and a table saw, should give you a lot of good wing spars. But the spar is not the source of a SPAD wing's strength.

The real strength of a SPAD wing is its (almost) triangular section, so long as the triangle does not collapse they approach being inpossible to break. The yardstick keeps the section from collapsing.

The exception is in the larger planes, where a joint is necessary to get the desired span. Then the spar has to be strong to prevent failure at the joint.

Gluing two thinner pieces side by side will not be materially stronger than a single spar of the same thickness. And spruce glued with CA will probably be weaker, since the CA doesn't work well on the harder woods. An easy strong spar is basically an I-beam. If you decide yu need a 3/8" thick spar, 1 1/2" high for example, cut two sticks 3/8"square, and fill between them with a 1/8" web, with the grain of the web running vertically between the top and bottom sticks. If you cut shallow grooves in the top and bottom sticks for the web to fit into it's even stronger. And use a good grade of white glue for assembly. Epoxy will also work, but you need to be careful, epoxy can add a lot of weight in a hurry, and be sure to use a very slow cure if you decide on epoxy. The one hour and faster epoxies are not as strong as they could be.

This should be enough to get you started, and maybe bring up some more questions.

Bill.