I have made several of large size for the 8 bore and 4 bore rifles.
(And one for a 2 bore)
They are not hard to make. I use a bolt of the correct size and thread, and that gets mated to a large piece of angle iron. If the tang must be long it's simple to weld some steel on.
The angle iron forms a plate to come out tight to the breach end of the barrel after the plug is threaded in tight. The top of the angle must align with a flat of the barrel. The rest gets sawn off and filed to conform to the other flats and you leave a tang protruding to the back. It's a lot of hand work, but not really hard to do.