With a PSK there are space limitations so it is necessary to think "within the box". A sling-shot is available by using the latex tubing already mentioned by Cyberraven. Also the wide rubber band mentioned as a waterproofing solution could be used. Get an inner tube from a bike shop for a fat tire (mountain bike) and slice a 1 inch width section out of it. This can be used around the tin in the place of the electrical tape (or in addition to it) and will work admirably for a sling-shot. The double advantage of the tubing is that it can be used to suck water from a seep as was already mentioned or to siphon other fluids as needed. The double advantage of the rubber band is that even if it breaks during usage you can fashion an ancient style sling-shot from the remaing pieces and the twine / fishing line / paracord / sewing thread already in your kit. Of course this style of sling can be every pit as accurate as a catapult type (after approximately 6 months of daily training ;-)) I imagine that even the condom could be used as an elastic for a catapult if you double it over. Not going to get much range but it could work.