Sue,

1)file a rectangular hole in the (tubular) metal a couple of inches from one end
2)flatten (hammer) the curved surface either side of the rectangular hole just enough that the hole is flat
3)hammer further the part of the flattened surface on the "long" side of the hole so it is just lower than on the "short" side
4)file the edge the hole on the "long" side to a sharp edge
5)shape a cylindrical wooden piece in the "short" end so that it just meets the first edge of the hole
6)file the curved surface of the wooden block just enough that when inserted it makes a "windway" a millimetre or so high
7)varnish filed surface of wooden block, glue in place and file to preferred mouthpiece shape

Some trial and error involved, but you shouldn't need any fancy engineering equipment

wr