If you insist on building it yourself, which I might be doing, I suggest having the exhaust up top of grow and intake for fresh air on bottom some where... As cold air is normally at bottom with hot air up top. You can also get a cpu fan and wire it to a 12v adapater and use that to get the air circulating this helps with temps and more sturdy plants.

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MY FIRST DWC
MY ULTIMATE SOIL GROW
A guide to growing in a neighborhood

like actually on the roof of the box? that's what i'm thinking.

Yea, and there is a really good diy on a kick ass homemade carbon filter.

You can use that to help for smell. I suggest getting another small box on top and attach filter to that that way the fan is pushing air throught the filter. But yea I heard of a muffin fan too suppose to put out alot of cfm, but some cpu fans put out extreme cfm...

__________________
MY FIRST DWC
MY ULTIMATE SOIL GROW
A guide to growing in a neighborhood

Also on the top side, its all about the space and what your working with. I had to sketch my plans up on pad to see what size i needed considering lights, space, container, all that good stuff. I am using mix 42 and 23 w soft white for flowering.

__________________
MY FIRST DWC
MY ULTIMATE SOIL GROW
A guide to growing in a neighborhood