You could try something like Holmz-Selfix 04430201.04 Slip-On Portable Shower by Holmz-Selfix. It can be found on Amazon.

Hi dagasd and welcome to AskMeHelpDesk.com. You piggybacked on an a 5 year old dead thread. In the future, before you post, look in the upper left hand corner for the date.
And they do make a 5 foot shower base (see image) that will convert a tub to a shower without changing the drainage. Regards, Tom