Shimano br-cx50
You can modify the sleeve if necessary, or use v- brake cup washers and spacers, to get the right overall dimension.
Or (shimano) br4600 shorter bolt slightly slimmer pad iirc, so didn't fill the space so nicely.
I’ve just used Odyssey bmx items (Slim by 4) but kept the washers from the old Shimano ones as they fit the recesses in the U brake arms better (the odyssey pads have V-brake style washers). Pads are very effective vs standard black ones.