I guess it was my site (garmin.na1400.info) but that feature is gone now. Basically it isn’t difficult, all you need to do is create a ‘feature’ layer and draw ‘vector’ features. These vectors are a ‘linestring’ of points that represent the area’s bounding box. These vector features can be loaded using a defining KML file in OpenLayers. Last, you need to add a click event to the feature layer which shows a popup containing the download link and/or extra info. That’s it, as said it is quite basic really but you need to get in touch with the OpenLayers way of thinking. Having experience with JavaScript might help.

For examples I recommend the vector drawing examples of OpenLayers. You can ask further questions here or join the Dutch IRC channel (I assume you speak Dutch/Vlaams?).