Cargo pants or BDU pants work well as they will have external pockets you can use for smaller things
the idea is to make a pack similar to a Veshmeshock http://wp.me/p1Ff3c-h3
Take the pants and lay them out legs facing down
tie the legs together and closed with a small piece of rope
Tie another longer rope that will form the straps to the first small piece with a large gap
Fill with stuff …
tie top like Vesh ….
and now you have rope and a spare pair of pants for no extra weight