I removed both the 92 mm heatsink fans.
I cut a big hole into the intake and exhaust plates. (See photo, the single at top right)
I cut a metal sheet from a cookie can and bend it into the right form for the venturi effect.
I put plastic foam to reduce noise.
I put the cover on.
Temperature is almost 10 degree Celsius lower than original singles.
Only if you bend the metal sheets in the right form. You can bend the metal sheets in different form to see that it has a big impact on noise and temperature.
After a few days I decide to change the fans speed by putting an electrical conductor to reduced the noise even further.
The white intake fan don't work with an electrical conductor, you have to change it with your own 120 mm fans.
How ever the black exhaust fan that come with the single don't have problems with an electrical conductor.
Almost no noise and still 2 degree lower than the original temperature.
And it is recommended to put the cover back on unless your setup also can cool the other components.
There are so many design flaws with the BFL singles that it can be easily overheated and catch fire without the cover on.