Have you tried to extend the tube at the focuser end , as from a 12" GSO I have tried this is where most of the scattered light is coming from. The diagonal is to close to the end of the tube.
If you look down the focuser and can see outside the tube than you may be getting stray light from the opposite side from the focuser coming up the focuser.
A general rule of thumb when making a Newt is to have the focuser cage length about 1.5 x the tube diameter with the focuser in the middle of this.
For portability they tend to be to short.
My Newt is a very open truss design with a long focuser cage and i don't have a problem with grazing light on the mirror , even with a full moon. I do how ever have a black cloth shroud for around the mirror for dew prevention.
Even my mirror cell and truss tubes are unpainted aluminium.
There is a lot of debate over baffles in a newt , but I won't go into it here but there is a great web site (can't rebember it-do a search if your interested) on thermal effects in a Newt with and with out baffles fans and flocking ect.(it has footage of the effects) Unless you have enough mirror clearance baffles can create more problems than there worth , and with excess clearance comes a smaller illuminated field.
P.S. most probable stirred up a can of worms with this , as it has in the past with posts on CN .
Pps get some descent optical black , not just matt black. It makes a lot of difference.
Here is a link for improving a newt
http://home.digitalexp.com/~suiterhr/TM/Topten.htm#01
Mark