As marlows sell AFM/Pod adapters for $40 for a RB30, and they are, yes, SQUARE! in the middle, i have designed, and made, my own, which restricts pretty much, no airflow, unlike these over priced rubbish from the shops... If you would like information, or are interested in purchasing these, please contact me via email... nate_dcruze@hotmail.com im not sure if i can make them for the RB20/RB25/RB26, they take a bit of measuring etc.. and i dont have one unfortunately