A 1 inch star fangled nut, with a little filing off the edges, will slip into a 1-1/8" steering tube. Stuff a little foam rubber into the tube first, add some epoxy glue, then slip the star fangled nut in place and add more glue to fill. Can't weigh too much. I've done this and even extended the steering tube length by 1cm, using a 1" copper tube coupling as a mold. The ID is 1-1/8", so it slips right over the steering tube. JB Weld epoxy recommended.

You can buy threads inserts in 6mm, no proble.