With the centre-section assembly complete, lay out the parts on the workshop floor again so as to size up the assembly mentally. The component parts of even the Titanic were once chalked-out on the workshop floor; and look what happened to that. I have called on a set of weights to secure the arrangement in position during this process. Knew I'd have a use for them eventually.
With this done, mark up where the bolt-holes will have to be drilled again in order to attach the centre-section to the perimeter frame. An advantage of square tubes over any other is that you cannot make a mistake doing this as they can always be turned through ninety degrees.
Entering stage right now are our the four-way tube-connectors, and there is only one way these will lie flat on the floor (which is technically the wrong way, reason being that we're going to want to terminate each corner with upward- and downward-facing stubs).
What we do here then is to secure a pair of connectors into only the lengths of tubing that are going to form opposite sides of the 'square' so we can slide them into place at a later stage. Make sure when you do that therefore with the previously-drilled hole where you want it to be when you've turned each length through ninety degrees to fit.
Finally beside the inevitable squeeze of silicone, rivet each connector in place during.
