Those are designed to mount on a hoop, so you'd have to either mess with your cajon or mount it on something else and then place it next to your cajon so that the piezo makes contact with the cajon itself. Not an ideal solution, I'd wager.
After messing with the idea and rigging up a little holder out of out clamps, i have concluded that this doesn't work. I can get the trigger to pick up the movement of the cajon and create a sound but even with the sensitivity turned right down it picked it up all over the face of the cajon. I pushed it in to the cajon and moved it away slightly but still no joy.
Oh well i am glad i didn't have to put any money and only an hour of time into this idea.