Water block for a triple hot end RoStock printer.  I have been working on this for quite a while (before the Kracken was released actually).  Fitting 4 extruders on the RoStock platform is difficult and isn't radially symmetric.   The benefit of using water cooling is that it takes less space and allows rigid alignment of the hot ends to each other... its also sounds better to have a water-cooled 3D printer;)  
I designed this to fit on the standard RoStock platform so all the holes should line up.  I have been using this in a dual head setup for a while, I will eventually move to triple head when needed and after getting everything else setup.

Instructions1. Print out the two drill guides  2. Clamp the first one onto the Aluminum bar stock so as to waste as little material as possible  3. Use a countersink/Center with a 3/8" wide head to locate the center hole location.  Don't just stick a drill bit in there it will wander and your locations will be all messed up... and your drill guide will be destroyed.  
4.  Remove the guide, drill the hole at 5/16" and use a bolt or some threaded rod or a bolt to clamp the guide to the bar stock.
5.  Now with the guide held with one hole use the center on one other 5/16" holes.  Remove the guide, drill the hole through and clamp with two bolts.  Now the other two holes can be safely located with the center without any worry of wander.
6.  For the smaller holes use a 3mm drill bit tip to mark the locations (or a 3mm/0.125" center if you have one).  7. Use the outer edge of the guide to trace the shape of the block  8. Remove the drill guide and drill through the large holes with a 5/16 drill bit.  Drill out the smaller holes to 2.5mm to prepare it for tapping at M3 9.  Now the fun part, use a hacksaw to cut out the piece from stock making the flats a priority for accuracy.  10.  If you have a lathe you can mount it to round out the piece or try an afgan lathe... 11.  Clamp the other drill guide around a side and once again use the center to locate the side holes 12. Drill the large holes at 11/32" to prepare them for tapping, be careful to not go too far but also be sure they connect with each other.  It may take a few tries, just be conservative at first.  13.  Tap the small mounting holes, and set screw holes M3  14.  Attach the fittings and tubing (note screw in fittings without extensions first (couldn't find swivel elbows at this size.

I went through a few iterations, an alternative/previous design used a slightly different hot end and was held in place with a nut on the top and bottom of the water block.  This worked pretty well but was a little more cumbersome to adjust the height and required a longer cold end meaning more weight and challenge in drilling the 4mm center hole.  The idea of using the set screws I took from the design of the Kracken (E3D).