My wife really wanted to try building a solar over based on plans she got on pintrest. I helped so I kind of ramped things up a bit.  It is just a cardboard box in a box. We lined the outer box with tin foil and layered in real insulation instead of crumpled newspaper. The inner box is lined in tin foil as well.

We had almost zero sun yesterday but when the sun came out the temp increase was sad. 65 outside and it got to 114 inside, 49 degrees.

Today we put a black aluminum plate in there and it jumped to 179 with the outside at 68 at only 10:45 am, a 111 difference. I decided to paint the bottom of the inside box black but leave the sides silver to reflect light to the shaded areas. I am hoping the temps will start to climb.

I'll check it again in 1 hour and see how we are doing.

while you are trying options, this funnel shaped solar oven from BYU is pretty efficient in northern latitudes...

http://solarcooking.org/plans/funnel.htm

230 at 11:15 am.

Are you familiar with sous vide cooking? It's low temperature water cooking of meat in a food saver bag. I bet  you could use solar to heat the water and get much more consistent results than a pure oven.  Steak is cooked at 130 or so for an hour+ and then seared over a fire.

Fwiw, a copy & paste from a post I did some years ago on another site.

Never used the parabolic or Fresnel approach, but we've used a couple box-style cookers for 6-8 years now. Haven't lately as we've been silly busy with stuff, but it does work surprisingly well. Made our first one out of wood and it worked great until my then-young son left it open in the rain for two days and it molded irrecoverably before I found it.

First one, made from wood scraps, a mirror, and plain styrofoam insulation. On a cloud-free day it would usually reach 370-380 degrees and more than once broke 400:


Used a table-leg bracket and eye-bolts for the mirror brace and adjustment:


Closed. Prop & all fit down inside for storage:


Don't have pics of the second one, but when I tried to make better I ended up making it worse. Made it larger and heavier, angled the top to face the sun more directly, and insulated it much thicker with cellulose blow-in type insulation. Turned out to be too much mass and it never got above 200 degrees. Lesson learned - mass is the enemy. Obvious once I thought about it, but I just thought about it too late.


Third one, I made out of an old fire-alarm control backbox. Sealed up the conduit entry holes with sheet metal and epoxy, insulated it with two layers of ductboard, and added the mirror & prop similar to the first one. Actually, the mirror is the mirror from the first one; not an exact fit for this new one, but close enough that I used it. This one reaches 300 pretty regularly, but has never broke 375 afaik. The construction makes it much more durable than the first two wood ones, but more mass to try & heat up and makes it heavier to carry around as well.

...continued...



Most-used pan is a simple aluminum pan with snap-on lid, painted with high-temp flat black grill paint:


Inside the pan, a cooking bag keeps the moisture contained. This is important on a flat-glass-top box cooker, as escaping moisture causes condensation on the glass lid, greatly reducing light entry and cooling the oven down below the cooking point. This is a ~3 lb roast, three potatoes, & half an onion. Usually add carrots, but for whatever reason, not that particular day:


Four & a half hours at ~250 degrees that day; slow-cooked like this, the meat is almost literally too tender to cut. It almost falls apart with just a fork:


I've been intrigued by the commercially-made box cookers, as they should be more efficient than my home-made, but never have taken the plunge on one yet.

Thanks for taking the time & trouble of posting all this OP. I've wanted to get a solar oven for a couple of years now, and I really look forward to experimenting with it.

I saw your post and bought the same pan and grill paint at walmart. I can't wait to get home and try it out. I bought an oven thermometer too. Thank you very much for posting. The black pan makes tons of sense.

Instead of using bags I am going to try the thin microwavable containers with the tight snap on lids. This way it's reusable.

I saw your post and bought the same pan and grill paint at walmart. I can't wait to get home and try it out. I bought an oven thermometer too. Thank you very much for posting. The black pan makes tons of sense.

Instead of using bags I am going to try the thin microwavable containers with the tight snap on lids. This way it's reusable.

I forgot to mention I used double pane glass for the top. Worked awesome!

I forgot to mention that insulation works by trapping air. You don't want or need tight packed insulation. You just need fluffy insulation to slow heat transfer. I thought I over did the oven in the OP. I should be playing with it again by 2:30. I hope it hits 300.

A dark pot/pan makes all the difference.
thought of using paint, but not sure about toxicity...

we use Granite Ware, and ensure that the moisture remains within the pot.
thought of getting dark Pyrex, but haven't needed to yet

I'd let the freshly-painted pan air out a good bit before putting it in the cooker, and then put it in the cooker (or your kitchen oven) empty a couple times before putting food in it.

I made the mistake of leaving it out while we were gone. The top pane of glass shattered from the heat. Not sure why. Now it's a single pane top.