I am unsure whether the trunnions are the semi circular holes in the carriage, or the cylindrical bits of the metal barrel which support the barrel. I am going to assume that the trunnions are the part of the barrel. (I checked. The trunnions are the cylindrical parts of the barrel which support the barrel.)
So, today I made some trunnions and silver soldered them to the barrel. In the full size original version they would have been part of the barrel casting.
But before that, I polished the barrel with a Scotchbrite type pad, impregnated with some polishing compound. And it made the barrel sparkle!
Then I attached the knob at the breech end, M4 threaded rod attachment.