If no one else can help I can sort it for you.I can indeed tidy up the slots so they are both 10mm-ish and square. I've got the correct tool for this freewheel, but unfortunately due to the mangled slots it is useless in this particular situation. But, on that basis...
Requirements V2.0: 35mm x 35mm x 10mm block, with a 6mm hole through the centre as per the diagram in post #1.
If anyone could help I'd be very grateful