We just went to the International River Crossing (between Michigan & Ontario, CA) and saw a ton of ideas for not only SWAPS, but how to carry them. Some were simple - shoe boxes, gallon baggies. Others were more elaborate - bucket hats, baseball caps, t-shirts.
But my favorite is the messanger bags - no-sew, handmade with either t-shirts, cut off jeans or fleece ties. I like the shirts & jeans because it relates to the "use resources wisely" part of the law. But, the fleece are way cute and can be custom made to reflect a girl's individual style, although any patter is likely to be covered up with all the SWAPS!
Here is a link to the t-shirt bags: http://www.artistshelpingchildren.org/kidscraftsactivitiesblog/2011/01/how-to-make-no-sew-tote-bag-craft-from-recycled-old-t-shirts/
And for the jean tote (you have to scroll down for this one): http://www.artistshelpingchildren.org/summer-beach-artscraftideaskids.html
Might I add that the Canadian Girl Guides really know how to SWAP!!!