Just get a piece of chipboard or cement sheet like material made from wood chips etc make a hole so it fits over the bolts, then put a bigger nut over the head bolt, arc weld it on the inside to the head bolts having one lead over the bigger nut and presto.

However like DD said use a proper hex not crown type of socket or ring spanner the full hex (flat sides unlike crown with teeth) will put even force all around them.

I had a worse situationr happen to me last year and used the arc welder in the same manner and just tack welding puts enough heat also at the same time.