Just for the sake of argument I'll reintroduce an old one that made me ditch the duct in favor of relying on the 140cm case fan and circulation fan....
If your case fan fails and your heatsink is covered you dont have much time before your CoBs get fried. Even with a little help from the circ fan an uncovered heatsink can radiate enough heat to buy you an hour or two maybe more.
...first... ...seal the duct..
...you can seal the duct using diferent sizes (or forms) of cover... i dont try to seal the duct at max values... ...if you use the fan on the upside the laterals sides are opened... ...if you use the fan on the lateral the other lateral side are opened...
...my inspiration are the cover / s with fan / s for hard disks or video cards on pc way... ...or the steveleds way but with cased heatsinks... not the tubes only... ...the sds lamps... etc...
...in case of fan fail...
...for these reason its used with protections... when the fan fail the protection / s stop the lamp...
...its rare the fail of an blower fan too... ...or on goods pc fans its rare total fail too...
...its ...but heyy its only my opinion...
pd... ...other crazy idea...
...maybe a possible way for obtain silent blower fans... ...reuse or recycling the silent fan / s for air conditioners...
pd1... ...other crazy idea...
...if the fan or the principal lamp fail... maybe the protection run the back up system with led tubes for fails on the fans or principal lamp... normally for high temps... and the led tubes helps with high temps too... ...or run other auxiliary illumination for deal with fail / reparation time...
...we can use the auxiliary led tubes for sidelighting too...
saludos