suggestions for a door that warps??

I built a oil barrel arch last year and it worked great, but this year I made some changes such as put it in a building, took the glass out of the door and insulated the entire door, painted it black. This year during our first boil, we had it really boiling and the door warped so bad it was a struggle to close it each time. The bottom warped out 3/4". When we closed it the latch would just grab and then as we pushed the latch down it would bend the door back in. I would have thought that putting insulation on the whole door would have been the right idea, but maybe i'm missing something. It warped some last year too, but never this much. I attached a picture of how it is insulated now, vs last year with the glass. It has AUF too. Any thoughts would be greatly appreciated. Thanks

A couple of questions, is that door a pre-made cast door, or one you made out of mild steel? How much of an air inlet do you have, it looks like just a real small round one. How full do you put the wood?
The wood should be split wrist size, and only filled to about 6" below the pan. Then it should have an air inlet of about the same sq. inches as the stack. I don't know how insulating the door made it warp worse but somehow it must have held or directed the heat to the door.
That being said, It's likely not a problem, the warp is letting more air in for a hotter burn. Just make sure hot coals can fall out and light something near by and cause some extra excitement.

The door is a homemade door from mild steel with an angle iron frame and a plate steel face. The draft that you see is small but I also have a squirrel cage fan forced draft that is plumbed in the bottom. I was running close to 20 GPH when it was warped up so bad. We burn seasoned pine, split wrist size. we have a timer set to fire the stove every 6 minutes and fill the firebox to the top of the door. There is no question it is burning hot, maybe too hot I guess. Do you think its possible that the heat is following the bolts that hold the insulation on back to the door and are doing something goofy that way. Last year we had round stock welded inside the frame to hold the insulation in so no heat could get transferred to the door except by the glass. Maybe by having the cutout for the glass it provided enough relief that the door could move w/o warping, now being solid it has to warp since there's not room in the middle to move????? What do you think.

Is it possible that it's the front of the arch that is warping rather than the door?

When you heat a piece of steel with that surface area it is going to warp as the steel has to expand somewhere. When the glass was there only the width of the frame had to expand and that was likely mostly absorbed by the fit of the glass. Dont think you'll get a solid steel door to not warp. Cast iron has a lower coefficient of expansion so might be an option.

So maybe my best bet would be to cut a large rectangle out and then rather than replace it with glass replace it with a floating piece of steel. Then insulate the whole door, but this would allow it to act similar to a wooden cabinet door with the panel floating free inside the frame. Then the main piece of steel will still have room to shrink and grow without warping the door. What do you think?

Weld some 1x1 angle on to stiffen the door in the direction it wants to bend. It would be a diagonal and make it symmetric with another brace to the other corners.

I use mild steel 3/16" thick, welded some regular nuts on the door then cut and screwed some threaded rod into the nuts. I then put 4" of insulation blanket on the door and used stainless fender washers and nuts to hold the blanket No warping after four years.

Thanks for the replies. I will try the angle iron diagonally first and see how that goes. I dont have the space in my firebox for 4" thick insulation but i can see that this would work too. I won't have a chance to make the change for a week yet so ill try to hold the boil a little cooler till then so i dont do permanent damage. Thanks again.