It’s easy to customize a Mundo. We used a jig saw to attach bamboo running boards for a dad in San Francisco. The oval holes allow straps to pass through for securing loads.
See the full How-To documentation on Flickr. Please note it is no longer necessary to use self-drilling screws into the Side Loaders of the Mundo, as the Mundo now has threaded customization points.
Here’s the finished product, with a child seat for his daughter: