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

My first Dingo wing, assembled ready for covering. The structure only weighs 6.7kg and feels very strong. I took my time building this one slowly and carefully, but now I'm familiar with the process the other three should go more quickly. I plan to use Oratex for the covering, as it's lighter than traditional materials. It's also a lot cleaner and less hazardous to apply, with no fire risk and no need for spray equipment. I'm sure traditional materials are fine if you're set up for them and have a bit of experience, but I'm starting from scratch. The UK agent for Oratex is TLAC, and I'll go on one of their one-day courses for beginners. They need a couple more candidates to hold the next course, so if anyone's interested please get in touch with them: https://www.g-tlac.com/oratex/