I found a good quality M6 stainless steel bolt with a smooth shank works very well.  I use washers & larger nuts as spacers to position the components on the smooth part of the bolt, and also a miniature G-clamp further along the edge of the components.  I clamp the paired parts together into a unit before fixing the first component to the spar, then keep them clamped until I need to separate them to fix the second component, later in the construction of the wing.  When fixing the second component I again use the bolt to ensure the holes line up, and leave it in place until the mastic has set