Remember to jump around while tack welding the perimeter to avoid warping or misalignment. The same rule applies when it comes time to fully weld the boxing plates in; give them some time to cool between welds if needs be. Next, dress the welds with a grinder to smooth things out, but not to remove too much metal. Avoid hard stones for grinding. They can quickly and deeply scar the surface you are trying to make nice. Try rubber-backed 36-grit sanding discs instead and replace them often.