Get it lined up good, bolt through each hole. Heat that plate a bit and work the edges to the frame, so there’s not much for gaps. Or clamp it
weld the inner most and outer most excursions, skipping an inch or so between each weld. Try to make the welds and the gaps about the same length
weld the inner most and outer most excursions, skipping an inch or so between each weld. Try to make the welds and the gaps about the same length