The only critical dimension, and it isn't really that critical, is the distance from mast to line hole. It needs to be further out than the straight run of the line, but not as far as to make the deck to spreader run vertical. About 1-1.5mm Aluminium strip is probably favourite being easy to work into the shape shown to allow fitting the same way as the original. Or brass, probably about 0.5-1.0mm. Either case, chamfer the hole to avoid fraying the line. Mostly a question of balancing strength against weight.
On other boats, I have used 3mm aluminium knitting needle. Reasonably easy to drill 1mm holes near the ends at the right distance to give the separation wantedand fit through the right size hole drilled through the mast.
