Hi Noel,
My plane isn't flying yet, but I talked with Kerry about various mounting options. And (unsuprisingly) the vent holes in the cowl may not make much of a difference in cooling performance, and mine is a A model waiex so I was left with the following three options for mounting the radiator:
* On the right side of the firewall (has the benefit that if I did want to vent the cowl I could, but I'm not excited about this choice because I presume that area has high pressure while in flight, so cutting holes might hurt by primary cooling)
* Mounted to the engine mount vertically (per the Sonex recommendation) - alas, that wouldn't work so great with my oil filter location. Also finding a location to mount the coolant reservoir becomes challenging in that case.
* I had a big area open just below the turbo, this is where I think I'm going to mount it. I'll use two adel clamps mounted to a bit of aluminum angle on the plennum, and a length of .025" with two screws in the firewall. It seems to me that if I close the two ends this area with triangles of .025", it would be pulling mostly cold air from the bottom of the plane (but I'm going to wait to try this hack after testing with a completed airplane)
To fasten the fan to the shroud I used #8 or #10 screws (I forget which size would fit).
Any feedback would be welcome!