Our house was built in about 1920, and when we replaced the windows several years ago, i saved the original jambs in a pile behind the shed. My wife recently had a family picture printed on 24" x 36" canvas, which was mounted to a 1 1/4" thick cardboard like backer. Her intent was to hang with no frame. I made a quick outer frame of 3/4" poplar, then straight-lined some of the old jambs on my home-made straight line jig. Had about 66" in length, so i cut 2 1/4" from each edge on the straight line jig. On each of the 66" long pieces, i marked the location for miter cuts to keep as close to continuous grain as i could. With that much width, i was able to simply pocket screw the corners to show her my idea (no glue at all). Picture below doesn't do it justice, and she loves it just like it is. (in the photo, the frame is just laid over the picture so she could see it--but the fit was spot on.) By doing it that way, the outer edge is still natural and weathered, as James mentioned.