The opening on either end of the cool tube is 6 or 8 inch.If your asking what type of flex to use you can just use the 6 or 8 inch flex.I would put a 6x4 reducer on the ends and use the smaller fan.I hope I understood you correctly.

Ga Grown!
GaGrown Reviewed by GaGrown on . Ventilation Duct Fan For COOLTUBE Question I'm hoping to use it to make a cooltube for a 400w HPS, alternatively a 250HPS. Will this do? These are the specs: TD 160/100. 2 speed: 2500/2100 v/min. Effect 35/23 W. Decibel 39/35 dB(A). Max. air: 160 m³/tim. Rating: 5