One really easy design "board" is a flannel-backed tablecloth. Lay out your pieces on the flannel side and roll it up. You can roll it around a pool noodle to help keep everything smooth. Easy to carry and to store. If you want to hang it up, you can use 3M sticky-back clips on a wall, or you can use pants/skirts hangers (clamp them to the top of the tablecloth and hang over the top of a door, on hooks, etc.). One friend even put buttonholes in her tablecloth and hooks it onto 3M sticky-back hooks. For one of my king-size quilts, I sewed two tablecloths together to accommodate the many squares that had to be in definite pattern. That thing got rolled up and toted all over the country (seriously!) as I worked on it while we traveled in our rv.
For a smaller board (up to 20"x30"), I've made little portable design boards out of foam core board and pieces of warm & natural batting using this tutorial <http://beeinmybonnetco.blogspot.com/2011/07/mini-design-board-tutorial.html>. You can make these any size you want...10" squares are good for laying out pieces of small blocks and keeping them organized until you're ready to sew them. You can stack them and transport them in a tote bag or box.