It would not be that difficult to turn something up from 12mm steel plate 2 thicknesses should do.
I have always been told not to try to weld cast iron although I have a couple of times with reasonable results. Those weights I was looking at with the steel sleeve might be OK Could weld only to the steel sleeve. You are supposed to preheat cast iron I think
I don't think there is much of a safety issue If the weld breaks the weight will just fall on the ground
I had blade disc fall off when I broke the end of the crankshaft off
It just fell on the ground . I didn't even notice until it appeared out the back of the mower
