After researching many CPU water blocks to use for cold transfer for a 40mm peltier I finally settled on the Alphacool NexXxos XP3 Light Acetal/Copper CPU water block.http://www.xtremerigs.net/2012/11/11/alphacool-nexxxos-xp3/
I did not buy these water blocks for how well they cooled a CPU but for the modification possibilities the above thread shows the stock pictures of the water block now I'll show you what I did to them to modify the 3 I bought.
My first observances was that a 40mm sized peltier would be pretty much fully covered by coolant flow over the copper base plate.
Which was even better when I discovered the dispersion plate was removable and my first intentions were to simply remove the dispersion plate and use the water block as it was but in the version 2.0 thread LiamG6 ran a flow simulation of the water block and it's cooling did not look good at all, then Liam suggested an epoxy fill and all my mental light bulbs lit up, his suggestion was IMO ingenious.
I used a rubber plug to keep epoxy out of the outlet threads, also not shown in the picture above I used the Dremel with a small round ball cutter tip and cut indentations strategically placed at the curve base around the interior perimeter. The purpose of those indentations was to give the epoxy seating points so it could not separate and come loose from the acetal top.
I then poured epoxy into the cavity around the center shaft hole, I used the original JB Weld epoxy not the quick set epoxy so I would have plenty of time to work with it and then let it cure overnight. You can clearly see the purpose of the rubber plug in the picture above.
Rubber plug removed the next morning, time for using the Dremel to clean up and fine tune the outlet hole tapering it a little.
Used a Dremel Tool to clean up the outlet and smooth it for flow.
All cleaned up and ready for the copper wall insert.
You can see the 3mm depth real good in this picture and the dremeling of the outlet area.
Did some crow foot etching on the smooth copper corners, I know it's not pretty but it works.
Added a copper flow block wall to keep the coolant from going straight to the outlet.
After the water block was reassembled the base was lapped flat prepping it for the 40mm peltier, it is used on the cold side of the peltier to transfer the cold to the coolant flowing through the water block.http://thermalright.com/product/true-spirit-140-power/
The hot side of the peltier is cooled by a Thermalright true Spirit 140 power heat pipe air cooler, it is a 360w TDP cooler.
Each of the Thermalright coolers was also lapped flat which was a time consuming laborious job because they have a convex curve in the base.
Alphacool water block mounted to the Thermalright cooler.
Assembly wired for testing.