Funny you should mention it now, I just wrapped up install on this this week and will be trying it out on the road for the first time today. I will say I maintained reasonable IATs on track with the HX just mounted to the frame with a fan behind it, no duct in place.Did you end up making the "3d printed mount and duct combo piece"? I am thinking of mounting mine on the side like this as well.
If you ended up making this, how has it treated you? Do you find it brings in enough air to the H/E? If it does, could I purchase one off of you?
Unfortunately I can't make another to sell, this is far from a bolt-on solution. To be honest, 3d printing probably wasn't the best solution here, but when you've got a hammer, everything looks like a nail.