One of the best ways I've seen (and i've done it too) is to take your lid and cut out the entire inside of it, leaving just a frame. Take a piece of hepa filter (blue on one side, light blue on the other. Has quite a few layers to it and is tightly laced together) and lay that over the box, then screw the frame back on. Make sure your screws go through part of the filter to hold it in place or else the suction will pull it in. Done some heave riding, muddy, wet and dusty and not one thing gets through the filter.