I thought of a NURBs solution for this! You could do it by breaking the cap up into a curve network (as shown below) and then create three birail Patches. Use this surface to loft the final object and then use the Round tool to put a nice round on all the corners.
EDIT: Do not attempt to attach the three patches you will not be able to maintain the shape.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675
Last edited by ctbram; 25-06-2012 at 02:31 AM.