There is a Sennheiser 2.5 mm to 3.5 mm cable which should fit, which is available from Addicted to Audio (Melbourne) for $20. It should be sturdier. I’m not sure about the 2.5 mm end connector size, it shouldn’t be too long, but it may be bit wide to fit into the housing of the P5. It would be possible to shave off some of the plastic with a Stanley knife to make it fit better: (particularly the lip, as you can see in the picture).
I will add that I have not tried this cable, so this is all just from net surfing
Someone else may have direct experience with the Senn cable and be able to chime in. Hope that helps some.
EDIT: If you give it a try, post to let the forum know if it fits or not.
Edited by Kernmac - 4/20/12 at 3:21am