Differentiating the potential from end-cap equivalent charges is so efficient at computing fields from bar magnets, that in less than minute, a laptop running MATLAB can computationally-optimize field uniformity in hundred-bar wagon-wheel (or spokes-and-hub) magnets. And optimized spokes-and-hub magnets have several advantages for hand-held and classroom low-field single-slice MR imaging (50-200 mT). Their frame is open, and they are easily assembled and scaled. We demonstrate several such magnets, from finger to wrist to infant size, and match magnetic field measurements to quadrature-based equivalent-charge simulation. We also demonstrate generating spin echoes using a spokes-and-hub magnet in a 200 mT tabletop imager.