How small can i hempy bucket be?

i am getting ready to try a sog setup in a computer case...im wondering how small of cups i can use with the hempy bucket

i only have about 18" of headroom

any tips / suggestions would be greatly appriciated

i was thinking of cutting 20oz water bottle in half and using them...any suggestions?

I have grown in a 20oz party cup Hempy style
Happy Growing
Green Dave

heres a pic of my grow box:

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