The "dohickeys" you want are called "offsets" and you can get them from
http://www.aswexpress.com art supplies.
They're hard to describe but the best way for me to explain is that they are little Z shaped pieces of metal that you screw into the frame. They extend inwards over the edges of the canvas holding it in place. The offsets come in various depths, depending on the depth of your canvas stretchers.