Milled copper would really help the centre piece, don't think it would matter as much for the hot plates. It's difficult to source in thicker sizes than 12mm, which makes the design interesting. Drilling & tapping G1/4" threads in the side won't work so well for one thing. Milling acetal is trivial relative to copper for another.
Not very worried about threads shearing. The acetal block is under shear from the bolts around the edge vs the pressure from the pelts in the middle, I haven't checked how deflection/shear fracture works out at the expected temperatures. Much less of an issue if it was 20mm thick copper, but I haven't managed to find copper in greater than 12mm yet. Occurs that a 3mm steel plate on both ends would reduce the odds of deflection, and that bolts nearer the middle would be useful.
Mainly posting a suggestion for avoiding condensation on tubing. 11/8mm tubing, with 6mm closed cell foam around it. Then wrap lacquered nichrome wire clockwise along the tube from one end, turn around and wrap the same length clockwise back down. The result is two wires going into one end, and two opposing spirals whose magnetic fields will pretty much cancel out. Put a layer of braid over it to help hold it together, then feed the whole lot down some 1" ID clear pvc. End result is 32mm in diameter and won't turn terribly sharply. The idea is pass a current down the wire and it heats up, some heat goes inwards through the insulation but most goes into the pvc tubing as resistance is much lower (about 10x lower) in that direction. The objective is to hold the 1" pvc at ambient temperatures even when the core of the tubing is at -40 or so (relative to ambient).
The idea is sound, it's extensively employed in industry though in less diy fashion. The other approach is jacketing the tubing with ambient temperature water, I'm pretty sure this is much more difficult to achieve than the above. The idea of course is that a 3 plate HX can be organised such that the entire outer shell is hotter than ambient, with only the two cold tubes exposed for condensation. If the cold tubes right up to and a short distance into the HX enclosure are warmed, that only leaves the socket still to go. Heated matt below it is simple enough, no ideas for above the socket yet.
On a vaguely related note, regarding stacked peltiers, based on published specs. A 12711-5M31-15CQ running at 6V will hold a delta of 20 degrees while moving 43W. It'll charge 33W for the privilege, so this pelt will output 76W total on its hot side. A 19911-5L31-15CQ running at 12V will hold a delta of 20 degrees while moving 85W, at a cost of 78W. With a 10W margin for error, this suggests that the bigger one at 12V can cope with the heat dump & transferred heat from the smaller one at 6V. This implies that these two stacked directly will move 40W through a delta of 40 degrees at a cost of (33+78)=110W or a cop of 0.35, which is pretty good for a 40 degree delta really. This is remarkably similar to the larger one running by itself at 15V. So while it's fair to say stacking them makes life considerably more difficult, I think it's misleading to write it off as impossible to use in computing. At least, if sticking to my conviction that two psu's for one computer is too untidy, then one at 12V and one at 6V avoids needing to use 15v.
Of course I guess one could just use washers and nuts..
