Use this mod to add EVA attachable struts to your vessels.
Attach the strut start point in the VAB/SPH, then use an EVA Kerbal (Engineer by default) to pick up and attach the strut, connecting two parts on the same vessel. Just pick up the part using the right-click menu, then hover the mouse over the target part and left-click. You can detach the part again using the right-click menu.