i make mine using a 10x10mm aluminium u section, the inner dimensionsa are 7mmx8.5 (1.5mm walls). i put a 1/4" (6.35mm) rod through it and thread one end using a coupling nut (just a 20mm long nut of the correct thread), which is the adjustment end. on the other end i put a 10mmx10mm piece of steel on the end, and attach that to the rod so that the rod does not spin when tightened (and so it has something to pull on) i attach this square piece to the rod by drilling a hole into the steel end, and through the rod where it will be, and then put a piece of a nail or bolt into the hole (should be snug in there) and glue it in. this stops the rod from spinning, assuming that you have made your channel accurately at 10mm, otherwise you can shim around the square endpiece.
i have been making my guitars with a body end adjustment, as it allows me to use the easily obtainable/cheap coupling nuts and also leaves more wood around the headstock, which can be prone to breaking when truss rods go through there.
here is a pic
adjustment end
locked end
this one i took a pic of was from when i was using grub screws, but decided that that was silly, and just a waste of time tapping the piece, as it doesnt need to be adjusted. i had recessed the point where the grub screw hit the rod after taking the pic, which made the grub screw flush and ensured it didnt spin.
guitar with it in
the guitar has a s curved end to the fretboard, so it had to stick out further on this design. on the designs i am making the nut only protrudes 4-5mm from the fretboard, which can be covered by the edge of a humbucker ring, and be acessed by removing the neck pickup using the ring end of an 11mm spanner. if you were to use a smaller rod then you would also have a smaller size nut, but i like using the 1/4" rod, as it creates a very stiff truss rod, acting similarly to carbon fibre rods in a way, but in the same space as the truss rod.
edit: just clarifying that the rod went into the aluminium u section from the pics, was sitting out to show how it went in. that should be easy to understand, but my description is probably pretty horrible as im very tired (somehow still feel the need to procrastinate about going to bed).