Dowel construction is neat, I like it. Proctor's E-III is built with 5/16" dowel as Don mentioned, and a small 5-point brass fitting is pinned between the joint. Cables are then installed, and the structure gets very rigid.

Birch dowel is commonly used, but I prefer ramin dowel because it's about 1/2 the weight of birch, generally straighter, and easy to work with. The E-III has such a long tail moment that weight savings are important on that model. I'll use a formed maple dowel for the front lower longeron, and might possibly use balsa for some of the rear crossmembers and uprights. Proctor's method of cutting the dowel is cool; a jig is made where the dowel is slid in from the side, then a 5/16" brad-point drill is used from the top to make the cut. Clever.

John