Here’s a traditional Swedish farm accessory for gunk-laden soles. The dimensions are not critical, but be sure the edges of the slats are fairly sharp?they’re what makes the boot scraper work. Cut slats to length, then cut triangular openings on the side of a pair of 2x2s. A radial arm saw works well for this, but a table saw or band saw will also make the cut. Trim the 2x2s to length, predrill, and use galvanized screws to attach the slats from underneath. If you prefer a boot cleaner that has brushes, check out this clever project.