DIY carbon filter size question

I have a 6 inch 400 cfm fan and want to make a carbon filter to fit it, and was thinking about making the dimensions 6x8x24. the 10 inch reducer and end cap are roughly the same price, as the 8 inch, and was wondering if making it 6x10x24 would make that much of a difference in performance. thanks in advance.

I would spend a bit of cash and just buy a filter. I made a diy filter from a guide on here and it didnt work, trust me it sucks trying to hide the skunky smell all the time i would say its worth 100 bucks for peace of mind

I agree with bbvc. For me, it was easier to buy a filter rather than make one. I got a CAN 2600 for ~$50; another $7 for the reusable flange.

Carbon filter are going to depend on more than dimensions. Carbon size, type, how well you pack it, is it prone to settling, etc. These are some reasons why I let the pros design & make it, then I buy it.
To try to answer your question, all things being equal but the diameter, the 10" should work better & last longer. However, it would offer more resistance to your fan.

How are your offering your measurements? When you say 6x8, do you mean 6" duct and 8" dia filter? This would be 1" thick carbon bed?
If yes, I still like the 10" dia filter, offering a 2" carbon bed.
My fan is ~300cfm for a small grow box. I believe the CAN 2600 (their smallest filter?) has a 2.5" thick filter bed.
Check out the CAN filter page, look for the filter that matches your fan & check out the specs and measurements. This may be a good reference for you.

Thanks guys. I guess I'll just wait and save up.