After using the displayport modded duct for a couple months, here are a few of my thoughts.
First, I don't feel even much of a breeze out of the vents of the duct. Perhaps this is due to the materials shapeways used or some factor of lack of constancy like that. Regardless, it works perfectly, except with most of the heat dissipation occurring through heat being transferred from the metal of the surface to the plastic of the duct, with the fan constantly cooling off the duct to maintain temperatures.
The duct fits on the side of the surface snugly enough to not move, yet not too much that it fights you sliding on or off. The cutouts for both the USB and the DP ports are more than large enough, and could actually be made smaller without any loss of function.
The main gripe I have on this prototype is the hollowing out of the arms that grip the surface to cut on printing costs. Within the first month of living in my bag, both of these have split at that joint and are in danger of tearing off completely. If these were not hollowed out, I do not believe this would have happened. I plan on using some gorilla tape wrapped around each of these to strengthen my personal alpha model.
All in all, the fan is attractive looking (I got it printed in red which matches my type cover), portable, and very easy to use in any situation, such as when using the surface on your lap. A friend saw me using it and posted to /r/surface where many SP3 users were interested in the idea. I told him to link to this thread. I have several friends who also use SP3s and really like the concept.
As much as Microsoft used the SP3 in their press conference for Windows 10 yesterday, it looks like it will be Microsoft's premium computer for at least several more months to come. I don't know if you're interested, but I feel like a well placed kickstarter to get industrial molds made for this (albeit maybe a slightly tweaked design) would find success. I mean, even an idea like this https://www.kickstarter.com/projects/1463948377/ears-for-the-surface-tablets
doubled its goal by fixing a design flaw in the SP1 and SP2. Just something to think about.