When I first started using the hood I thought everything was fine, but after using it two or three times I did notice a downward draught, that was cooling the bed slightly.
My first thought was to seal all the gaps but this did not help. I then tried adding a sort of extension skirt around the bottom of the fans. This seemed to do the trick.
I might be wrong, but I think there are all sorts of turbulent air currents and this helps still the air. I can still feel a slight draught but before I added the skirt,  it actually felt like the fan was blowing down!
I am beginning to feel more confident about 3d printing in general now. Usually once they have started printing I walk away, with only a periodic check.
Some prints have been >5 hours after the bed had come to full temperature.
To test out my new hood I printed this catapult http://www.thingiverse.com/thing:391508 The amazing thing about this catapult design, is that it prints in one piece.
This was my second attempt to print it. The first came out okay except the bucket of the catapult, being printed on it’s own, started to move. I saved it by holding a screwdriver and it sort of fused together.

I could see all it needed was some supports so I modified the file look like this

Added support pieces are coloured