thats easy just do this
Old Soap Slot
An unbroken chain
Make a bar of soap with an off-center slot running down most of its length. This slot is for the last bar of soap when it gets down to a nearly unuseable size. Just slip in the used soap and continue, rather than trying to stick the old soap to the outside of the new.
The main problem I see is that you might carry the same tiny bit of soap from bar to bar, never using it up. I'm not sure what the result of this would be.