I've now modelled the cooling ducts. The airways themselves are not optimised at all, but it should serve as a good starting point. 

I also did this to see if there was anything else I might need before I bought the AliExpress BOM, since I've found it unrecommendable to buy components before the assembly file has been completely modelled.

I'm planning to self-tap M5 screws into the 6028 fans. Here's what it looks like with the fans inside:

Hopefully, the plastic being 3mm away from the hotend is good enough for it to not melt.