A bottlecap for a 28mm PET bottle. Like common standard 1l, 1.5l and 2l coke bottles.I've included the source file (Sketchup 8).You can print this solid or with 20% infill (as in photo, left cap).
I usually design things in Sketchup to print stuff on my UM1 or UM2. The designs are always accompanied by their source files for editing and a collada file for printing but if you want a particular format, just ask.