28.6 sounds about right.
Standard diameter is 25.4 and oversize is 31.8. I remember the Alpinestars "Mega Lite" (yeah right!) bars/stem being somewhere in between.
If the steerer tube method doesn't work, try a section of copper pipe. I've done it this way in the past when I had to fit some Azonic Chubbys to a Marzocchi direct mount stem.