The design could be easily lengthened for the K16 to create a longer block right?
Some have plans to use the heat sinks to help give rigidity to the boards when they are daisy chained together.
Off the shelf here you can get the aluminum sq tube 200x50 in lengths of 6.5m
Cannot get flat bar that wide (that i could find) but you can cut sheet (was looking at nesting and getting it laser cut anyhow).
K16 is 100x100 so you could fit 2 across a 200m tube with no tolerances. So 2 mounted either side is 4 per 100mm. 6.5m length could in theory hold 260 boards. My concern here is 260x40w=
10.4Kw can enough water flow through the block to cool that many boards?
I do not know how good the heat rejection on the tube will be yet but a am comfortable with my 38 x K16 on one block.
The K16 Modular Design is a 4x4 16 boards (400mm x 400mm no tolerances). You could stack two 200x50 water blocks one on top of each other?