Wish I had known about these threads before I made mine.
What I did was buy an A5 planner (I don't know if B5 would fit, but the only suitable size around here is the A5, which was a bit big). It was one of those planners where you can remove the pages inside since it's just tucked into the back cover. I specifically looked for something which is soft, but has some sort of hard board on these inside so that it protects against the screen from accidental jabs.
Then I fixed my K3 to the back using the adhesive tapes from the 3M hooks (since they promise not to destroy the wall when you take them off ;D)
And finally I put a pair of velcro buttons so that the cover doesn't keep on opening when I don't want it to.
It's pretty fun actually, and I'm planning on painting some sort of design on the cover (using nail polish, haha). The only downside is that they don't sell the tape and the velcro buttons individually. Don't know when I'm every going to using the rest of it...
All up it cost about AU$25 with a 2011 planner, 16 tapes and 20 hooks, and 11 pairs of velcro buttons left over.