I have been attaching a fan to the end of my tri-sli'd gpu's since I first got my 480's. The 3d printable bracket I modeled for the 680's doesn't work so well with 780's in an 800D, so I hadn't been using it. The 780's are just a tad longer, and the fan would have been right up against the sata connector cover on the 800D.
After playing Farcry 3 for a few minutes on the new setup, I was surprised to see my top two cards were hitting 79 degrees, while my bottom card never got over 60. Coming from the cool 680's, this wasn't good enough, and the cards where obviously suffering from suffocation. I modeled and printed a new version that tilts up and draws in air from the empty 5.25 bays. Bottom one still stays as cool as ever, and the top two stay in the mid to high 60's.
Use the best settings for overhang on your printer. These come close to 45 degrees, but they are printable without support. I used 1.75mm Protoparadigm PLA on my M2 printer. Mount using short m3 machine screws.
This Thing has worked for me, but your mileage may vary. Their are too many variables on everyone's computer setups to give a guarantee. Try it out and post your results. Remixes are more than welcome.