I reviewed a whole bunch of these on Thingiverse, and the best combination of simplicity and effectiveness appeared to be this one:
…but you want to print these at 0.1mm layer height, or less, and that’s difficult with the model above. So I tweaked it a bit:
NOTE: The WordPress authors can’t be bothered to implement safe/secure uploads; instead, they block ALL uploads (which is very stupid and doesn’t solve ANY security problems). This file has to be renamed when you download it. Stupid, stupid, stupid programmers.
Changes
The original model has two small circular screw-holes. Makerbot printers can’t print these very well – Both the official slicer, and the main commercial / open-source slicers get it wrong and print the holes in a bad way (untethered; it’s an obvious heuristic to fix, but no-one has implemented this yet). Net effect: the holes slide about during layer 2, and usually detach, and ruin the print.
To fix this, I cut a slice out of each hole, making them into hooks. This follows the design of Makerbot’s original fan duct, and makes the print easy to do even at high speed and low layer-height.