You can easily make an octagon frame from wood if you have a mitre saw. You can build an octagon almost as easily as a square frame; the construction is the same, allowing that you have eight sides instead of four. For an alternative to the everyday picture frame, just get your mitre and start cutting. It's best if you start with a flat style of moulding or trim; then once you get the feel and the angle correct, move on to profiled mouldings.
Set your mitre saw at 22.5 degrees. Cut 8 pieces of the pine lumber at 6 inches measured from the longest point to the longest point of the mitre. This measurement should give you an approximately 12-inch inside frame opening when finished.
Scrape off all the remaining splinters so that the edge of the mitre is clean. Lay out all the mitres on a flat surface in the shape of the octagon. Bring all the mitres together. Place the strap clamp around the octagon frame and tighten slightly. A strap clamp is a long piece of nylon strap with a handle that tightens the strap. You can find these at any home improvement store.
Check all the joints. The strap clamp should even out and close all the joints. If the joints do not all close, adjust the mitre saw to sharpen or lessen the angle. Barely trim one piece, then put the frame back together and tighten again. If the frame still won't close all the joints tight, trim another piece, fit again and then check the frame again until the octagon is tight all the way around. Keep the order of the pieces.
Place glue on each mitred end. Quickly assemble the octagon, wrap and tighten with the strap clamp. Tap on each corner to make sure the octagon is lying flat and that all corners are aligned. Let dry overnight.
Take off the strap clamp. Turn the frame over and hammer one corrugated nail into each mitred joint. Corrugated nails are short, flat pieces of metal that are sharp on one end. You can find them at any home department store. They are also called "spline nails."
Once your frame is dry, you can trace around it with heavy-duty construction paper, cardboard, or 1/4-inch plywood. Cut out the template, run a bead of clear silicone around the back, align and lay the frame on the template, and let it dry for a backing. Since it would be costly and difficult to cut glass to this exact shape, it's not recommended that glass be installed on the front.