This is a 40mm fan duct for the Smartrap Core / Smartcore 3D printer. I designed it around and have checked it with my V1.2 printer that has an E3D Lite6 hotend. I think it should work for E3D V6 too but can't check for you.
I used a 3mm single layer brim to help keep the corners down. I also managed to print a usable version without a fan and just a bit of blowing for the slot bridging towards the end!
The CAD for this was done in Onshape, where this is a public design. You can find it if you search for "Smartcore V1.2 Duct" and look in Part Studio 3 (the others are just old attempts).