Drop Stuff, part of Stuffit, can be changed to create .sit files by default, but only in the newest version. Alternatively, you can right click on the file and navigate to the contextual menu item for Stuffit and select .sit from there.
I can't find this option - I have the newest version of the standard Stuffit 2010 (not deluxe). About half the files in the EQMac section of EQInterface are .SITX however so I reckon they have just removed the file type from the permitted list since there have been no EQMac uploads since 2006.
Why even bother with eqinterface.com
Because EQInterface is the central resource for things like this. Folks have heard about it. It was the first place I went to when I wanted to see what UIs were available for EQMac. If I change hosts or stop playing in EQ in the dim and distant future, Sars4mac would vanish if it was on my own web space.
If EQInterface give me the runaround however, I will just host it myself and attempt to get a refund from Stuffit. Users of older Macs will have to work with a .ZIP archive then.