I built a leg-strap knee key for portable CW operation.  See attached photos…

I made the plate from two plastic outdoor electrical box covers glued back to back with a slot of the foam insulation cut out to pass the canvas strap. 

The strap is a military style belt cut down to fit my thigh.    I got it from ebay.  The end I cut off was damaged - I had gotten a free replacement for pants-holding-up use so this one was a bonus.

Very comfortable and seems to be big enough for nice sending.