Picture of Another portable duct tape idea
This is a modification of another Instructable: http://www.instructables.com/id/Pocket-Sized-Contest-Always-Have-Duct-Tape-in-You/
Basically, wrap the duct tape around a pen and don't cut the pen ends off.  That way it can be placed in the pen holder or compartment of a knapsack and be easily retrievable.  Also, the ends make it easier to roll the tape on and off the pen.