Cut off a 21-in.-long board for the shelves, rip it in the middle to make two shelves, and cut 45-degree bevels on the two long front edges with a router or table saw. Bevel the ends of the other board, cut dadoes, which are grooves cut into the wood with a router or a table saw with a dado blade, cross- wise (cut a dado on scrap and test-fit the shelves first!) and cut it into four narrower boards, two at 1-3/8 in. wide and two at 4 in.
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.