take door off - set oven on its back - take nothing out - measure 2" deeper than largest pan you will use - build cardboard box to fit inside which brings base of oven up to that level - paint top side black - then cover with tempered glass or oven bag - build reflector frame from cardboard and set over oven - make 4 appropriate sized reflectors from cardboard and foil - use foil tape to mount on reflector frame - cut and bend metal hangers to hold reflectors at best angles - if reflectors have the corrugation running horizontal to oven then insert bent ends of wire hangers in edge of reflectors to adjust - cook as in any solar box cooker - it is so well insulated that it will serve as a retained heat cooker if covered with insulating material after food has reached cooking temperature *
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Yes, you could simply strip off all parts leaving only the metal cavity, block off the cover where the magnetron was against the cavity with metal too, then paint the entire outside surface that sun shines on with flat black heat resistant paint. To improve performance slightly you could leave the bottom unpainted but put some insulation on it so areas not exposed to sun are not radiating heat to the surrounding air as much, or instead you could position mirrors around the oven to cast more sun onto the sides of it (also painted flat black).
You might find that the typical plastic door gets warped over time, it would be better to replace it with a black painted piece of sheet metal that seals as well as reasonably possible. If you can manage to make the entire thing an air-tight seal then you may have better results from the small pressure buildup.
However, after all this I must say that it won't be very hot. The ideal in a microwave is near maximum usable interior space per exterior dimensions, while the ideal in a solar oven is minimizing the interior or hot zone while maximizing the exterior surface area that catches sun.
Towards that end, the smallest volume compartment you deem acceptible for the quantity of food would be better, with the surrounding area using far higher surface area mirror-like panels to focus the reflected sunlight at that area, so you could use a microwave oven cavity for a lot of food but would need a proportionally larger, relatively huge mirror array to heat it enough for safe cooking temperature of anything perishable.
In other words, you would be better off starting from scratch than using the microwave oven at all.
Thanks for your information. I was thinking that the microwave would already have good built in insulation and might work well for solar cooking. I appreciate
your detailed response. You've given me a lot to think about.