I received my 20 300mm threaded steel rods with lots of nuts and washers so I set about making some corner brackets to hold them all together. I don't have a pillar drill so I just had to drill holes with an electric hand drill and a set square. This worked reasonable well but the holes were very rarely at perfect right angles. To 'fix' this I used a reamer to enlarge the holes, allowing the steel rods to line up straightly. This would not work in the standard Darwin design using un-threaded rods and grub screws. However with this design it does not matter if the hole is a bit bigger, as long as the blocks themselves have right angle sides then the rods will line up when the nuts and washers are tightened. While not elegant, it works.
I think 300mm rods may have been a little small and will give a very small [but usable] print area. I probably would have been able to get away with some smaller motors too as the skate bearing axes require very little force to move.