I made one out of box-section....
60x120 for the main lump. then three lengths of 20x20 to pad out the width, and to replicate the cut-out in the crossmember.
From memory, its 1.5m long
Use the 60x120 on its side, and weld two of the 20x20 lengths against the face that will be the rear of the cross-member.
Chop the 3rd length to fill in the remaining space, but have a 45-degree face to replicate the cut-out.
Plate one end of the new cross-member, then dripple a small amount of oil (any sort) into the voids before plating the other end - the oil will help stop rust forming internally)
You'll need to move the body-mounts across (or, in my case, make up new sides for the existing rotten mounts)
Weld in as per any other cross-member.
... oh - and 3mm is PLENTY... the original is only 1.5mm!
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