(which is still going strong...although I did have to replace the rubber band)
here are a couple of other hacks.
A small scale watercolour and drawing set. Just a piece of elastic to hold the watercolour field kit and the artist's coloured pencil in place. I can also leave the watercolour field kit off, if I like. For the non-metric reader A6 is close to 4 by 6 inches, okay for my large coat pocket or the cargo pocket in my pants/shorts.
I like using either wax based coloured pencils (artist's quality or even children's coloured pencils) as, unlike graphite, charcoal or conte, you don't have to worry about smudging.
Hack number 2, uses a bend back clip and a little duct tape (taped to the inside of the clip) to hold the pencil (in this case, a watercolour pencil) with the sketch book (A5 or roughly 6 by 8 inches).
Hope these hacks might be useful to you. Good sketching!