Here's before and after pics - tape applied to the area just in front of the OLPF to cut the flare
No mattebox or flagging was used on either shot.
THe lightsource just out of shot is a twin-LED flashlight - hence the two flares.
The out of focus blob is the stand the flashlight was sitting on.
The reduction of flaring at the very top of the frame is due to shadowing caused by a tiny vertical surface near the OLPF, that "flags" the reflective surface.
You can see that the flashlight caused virtually no actual lens flare. The flare is basically 100% due to bounce off the OLPF mount. Only the vaguest, soft blue tint remains.