Add a reflector and a heat plate.
Remember the television commercials for the miracle plates supposedly covered with a super conductor that were meant to be used for thawing frozen foods? There were several different names, and they actually worked pretty well. Place one inside your oven where the sun can hit it directly. The suns rays will be absorbed better, raising your heat quite a bit. A stone plate in bottom already heated before beginning the actual cooking
cycle will help sometimes. One of the large magnifiers can be used to focus higher heat onto either a black pot or the conductor covered plate.
solar oven. Get a piece of sono-tube, or whatever it is called where you are located. This is the thick cardboard tube used as a form to pour concrete piers. Short pieces for things like deck
post pads and such are available at hardware
places. Cut about a 10 inch deep piece. Cut a second piece at about 9.25 inches. Slit the second piece down the side and slowly trim it down until it will get small enough to slide inside the first. You now have a fairly air tight cardboard tube about an inch thick. Cut a round base with a groove for the first ring to sit about .25 into it. Slide the second one into place. You now have to make a top with double glass that has a .25 lip that fits into the recess left by the inner circle being a little shorter than the outer. Aluminum flashing to make a reflector. Taking time at this point to make brackets or ways to adjust the reflector will be appreciated later.
Set in sun. Use reflector to reflect light into the oven. Use magnifier to raise heat and add to efficiency. One of the large 12 inch magnifiers that come with a ring light and an adjustable stand works perfectly for this.
doesn't do very well being heated to high temps and bathed in steam time after time. The cardboard tube lasts longer and is easily replaceable. Make the tube two inches taller and add a gray or black round landscape stone to the inside bottom as a heat sink.
You can get fancy from there if you wish. Deep black aluminum or cast iron pan with two or three layers of the tube around it and with a stone under it, gives you isolation from the carbboard.
Sorry so long, but I spent a summer playing with re-inventing the solar oven one year!