Actually, when I researched a solution for my office, I was amazed at how many type of custom-ready window conduit solutions there are on Amazon. For under $20, one can get an entire kit that has all the pieces one might want in sizes ranging from 3-6" and in some cases, oval shape. Thanks to the price falling for portable air conditioners, they now have horizontal, vertical, and even solutions that will fit inside a inward opening window with zipper seals.
One is only limited now by one’s imagination. And of course… I suppose… if one really wanted to… one might even 3D Print something… 
