This could often be a person, for example a friend or confidante, who absorbs your stress when they listen to you go on and on about work or something.
Alternatively a stressel could be a literal vessel, such as the pint of beer you eventually drop that guy's mobile into after it's been ringing incessantly for the last half an hour. Try it. It's fun.
"Who's been using the bird-bath as a stressel again?" Said Giles as he fished yet another cowbell out. (Possible Emmerdale usage)
"Pass me that stressel, I need to be sick." (Possible Sex and the City usage.)