Driving bits.....

The horsebitshop do a 3 slot liverpool up to 7" if that's any use, they are straight bar, but if you have a friendly farrier with a furnace I'm sure he'd be able to put a curve in it if you bought a size up; they are £25 if I remember rightly.
 
I'm not sure heating a S/s bit is a good idea, Liverpool bits are designed to be double sided, me, I would not want to think I had weakened a bit, it is kind of of vital in a harness horse.
Some driving people use a butterfly pelham which is kind and looks the part.
 
Top