how to make a 3" to 6" adapter

i am trying to get my 3" flang to connects to my 6" duct and inline fan does anyone have any ideas of making a airtight adapter thats goes from 3" to 6" trying to not buy one

use a lot of duct tape?

try a small plastic container,folgers plastic coffee pot comes to mind, and just put a hole in each end th size you need. Like the poster said before, lotta duck tape. But it should still work, and save ya like 12$
Good Luck