...Personally, I'm a bit inclined towards a minimum effort with best possible result, which is why I pointed out that an Accelero only needs a little modding to fit...
Actually it would need a lot of modding because there is only ~15mm between the edge of the graphics card and the side of my case, but the S1 sticks out further than that (I had to cut a hole in the side of my case
to make a Thermalright V1 Ultra fit).
...Anyway, I imagine an Accelero might be able to mount the way you want if you unbent the heatpipes (assuming the mounting holes are arranged in a square shape). You might have to chop off a bit of the ends of the Accelero's fins anyway, as it might be longer than your case is wide.
I had considered that, but I think it would hit the motherboard, and there is no way to cut off enough to prevent that because the heatpipes are there.
You would need some duct. Plus, the possibility to ruin the heatsink.
Accelero L2 Pro fits and is almost silent.
Ducting is no problem (although I suspect it may not be necessary because I don't need any on the CPU, which is already passive). I am perfectly willing to accept the risks (although this does mean that I don't want to spend too much on the heatsink, in case I break it). An Accelero L2 is no good because it's what's on there already (it's the non-reference Sapphire 4770
), and it's not quiet enough.