Here's a view of the cube from the underside. In order to make up to some extent for not having overlapped it and riveted it to the frame too I have at least sealed the gap with silicone. I prefer this to glue for prototyping as it's surprisingly adhesive in itself, and unlike glue it allows builds to be dismantled for modification... as we saw already.
We'll finish off the topside in plywood, because I've run out of sheet alloy and won't bother buying more in view of the spectacular waste those cut-outs have created. At the same time it's a whole lot easier drilling ply at larger scale than it his sheet metal.
That said I found a workaround to stop large drill-bits chewing up the metal, by using timber clamped either side. A prime consideration however is that I selected three-quarter inch and 19.5mm metal bits, neither of which allow enough room for the con-rods to slide in, plus wood-bits at that scale are around five times cheaper.
