18mm side access spanner?

Ive got a problem with a fiesta zetec-s rear beam, there should be a kinda nut welded to the inside of the beam on each side but one was broken off by the person removing it.. ive bought a nut to put on the bolt but I cant get a spanner on it to hold it as it'd need attacking from an angle.

this sort of thing would have been ideal (half moon spanner) but it needs to be open ended and I cant find a solution anywhere.

Anyone any ideas?

My other thought was if I could get an M12 threaded nut with in a smaller size so there was rooom to get around it but not sure if thats obtainable?