I am becoming more intrigued on how the soldering is going to be done as it looks to me that getting all those plates to line up and pressed into place is going to be a very tricky job, which will get harder as you put more plates on and the thing stiffens up, unless everything is absoutely dimensionally **** on. Have you considered making the holes in the plate a fair bit larger to make fitting easier, then using copper washers to bridge the gap? You could offer a plate up to the tubes, clamp it up, put the washers on, draw round each washer, take everything off, solder the washers to the plate in the exact same position, so that when you offer up the plate again it should be tailored to the tubes. Just have to use washers which have a inner hole dia close to the tube OD. With the plate in place solder the washers to the tubes, the washers may well unsolder themselves form the plate while you do the second solder run but if you clamp the plate and do them one at a time it shouldn't matter. Then repeat this for the next plate.
at JonJ's mains AC idea! In words of Sergeant Wilson "Do you really think that's wise sir?"
edit: Just thought how my idea could be improved. Why not fabricate your own squaure washers using the flat plate you already have. Cut the plate into squares, coat one side with flux and seed with solder so you have a square of copper with a film of solder on one side. Then drill a suitable hole in each washer. Pre-soldering each washer removes to need to put the loose washer on and draw round it, you should just be able to put the pre-soldered washer on the cooling plate as positioned on the tubes, and blast it with your blow torch. The solder may even run around the tubes as well, though it will probably still be necessary to put an additional ring around each tube. The key benefit of this is you can enlarge the holes in your cooling plates so that they will be more forgiving to any measurement errors.