Whilst looking around at the various mods done to K2's I discovered the following balun.
It is a small efficient QRP balun designed some time ago by Charles Greene W1CG. see www.njqrp.org for further details, including full building instructions.
My K2 is setup on internal batteries with it's main goal to be a portable radio. As I use mainly doublets through a 4:1 balun I thought building this inside the K2 would suit my needs perfectly.
I purchased the toriods from www.qrpproject.de you can buy these ready made but I wanted to build it! The toriods purchase were Amidon FT82-43.
As I had already performed a mod to add a separate charging socket to my K2 I knew I would have to move this to allow me to add the balun.
The balun consists of two toriods wound with 6 turns on way, a cross over and 6 turns the other way. Then wired together to give 4:1
Winding was pretty easy compared to the many toroids in the K2