First, here's the completed unit - slightly modified since the previous photo to make it a little more compact, and to reduce the length of 'throw'.
Here are all the pieces that are required for each unit. I cheated slightly by cutting the 8x2 flat strip into 3 pieces using a razor saw but there are 'real' pieces made to these sizes. The colours obviously are down to personal taste! In addition to the Lego you need a length of hollow brass tubing (mine is about 2.5mm dia with a 1mm dia central bore (the length depends on the thickness of your baseboard - it needs to be long enough to extend up to just below track level, in my case about 4cm).
1. Using a round needle file, file notches in adjacent faces of the 12x1 pieces so that when placed together they provide a tight fit for the brass tube.
2. Insert a connecting 'pin' into either end of one of the strips.
3. Clip the two strips together.
4. Insert 4 pins into the outer faces of the strips as below. These act as stoppers limiting the 'throw' of the unit.
5. Add reinforcing pieces to the underside at either end.
6. Add reinforcing pieces on top.
7. File the tops off the two 2x1 pieces.
8. Clip the 2x1 pieces to the protruding pins at one end. These reduce the throw of the unit slightly more (may need removing once everything is hooked up depending how much 'give' there is in the whole assembly.
9. Insert the brass tube. The exact height can be adjusted once the unit has been installed, and the rod fixed in place.
10. Drill guide holes for screws in the four corners of the baseplate.
11. File the tops of the centre two strips of the 4x4 piece (the exact amount will be determined by the ease with which the moving bar slides back and forth once the unit is fully assembled - it doesn't want to be too loose or too tight).
12. Add te 6x1 pieces on either side of the 4x4 piece.
13. Add the 6x1 flat pieces on top of these.
14. Insert the sliding bar.
15. Complete the unit with the 6x4 piece on top.
Here's a close-up of the slot drilled through the baseboard adjacent to the tiebar. The brass rod attached to the TOU will project up through this hole to surface level.
And the hole drilled in the front of the layout from which the operating bar will protrude. Once everything is in place this will be covered over and disguised by point levers/cranks etc.
Next job - fit the TOUs to the underside of the baseboard. Then install the operating rods (still need to think up a locking mechanism and figure out how polarity switching will be managed). Finally, install the tiebars and surface level operating rods which will locate into the top of the brass rod.