If you have a local blacksmith/farrier or even engineering firm you need to get a length of square tube (?) and cut it to the right length and then drill it for you. The one in my trailer I think is about 2inch by 1inch. You will have to measure up the gap where the bar would slot in.
I've got an old Bahill which is based on the Rice design. My local trailer man cut down an Ifor full breast bar for me and sorted out fittings. Think he charged me £15.