1. Mostly the bottom of the suitcase is not flat (due to the wheels and telescopic handle). You can fill up the cavities with small things, and then cover it with a piece of hard board before laying the trousers flat. 2. Remember your suitcase can travel even upside down. So use the elastic bands. 3. If the elastic bands are not strong enough, and there is space at top, through in an inflatable pillow, which you can deflate later to accomodate shopping.
