You can trim a miter quickly and accurately with nothing more than a sharp hand plane and a simple jig, like the one shown in the drawing. We built the jig out of medium-density fiberboard (MDF), making sure that all of the cut edges were perpendicular to the faces.
Clamp the jig in your vise, and then place the work piece against the appropriate guide, allowing the mitered end to protrude just slightly beyond the plane guide. Set your plane for a light cut, place it on its side as shown in the photo, and make a pass across the miter.
As with any method for trimming miters, this one calls for frequent testing of the results. Depending on how much material you need to remove, test the fit after one or several passes.