With the arms teed up on that back-plate, use a 10mm (confusingly that drives a 6mm shank... it's all about the head-size) box-spanner to hand-tighten the joints. Normally I save the final twist until the assembly is complete, as it provides for a little leeway for the inevitable jiggery-pokery involved.
I just realised too (or else I'd forgotten from previous, which is one benefit of sharing knowledge among the matrix) that rather than slip out that template from underneath we could just have used the second of the pair of sheet-alloy squares to place on top.
What I did again here tho' was to slip that square out from beneath and set it up on top with the benefit of the set-squares to keep things true. When I say true, we're not working to microns here because ultimately it doesn't matter: unique amongst the designs of flying machines drones are simply self-levelling devices that don't care as to how and where the various motors and propellers are mounted.
With things broadly looking just-so, therefore, we hammer four more rivets around the perimeter to hold the top-plate in place. With this done we can all relax, maybe over a cup of tea. Then add a rivet in each corner, being careful to avoid the internal bolt else removing each one temporarily to allow. I generally then add a fourth and fifth rivet at each side, though this can be done at a later stage once the general outline is secure.
