Batch box lids--what are you using

Hi everyone, a recent post made me curious as to what everyone has used (successfully and unsuccessfully) for their batch rocket mas heater lids. This would be the material used to span the box of the rocket mass heater.

I can attest that a thick cast iron griddle has been working well for four years of burning in an 8".

A friend has a sheet of steel on his 6" batch with a layer of superwool protecting it.

I feel like I've seen bricks being used for smaller systems, but for the 8" brmh you need to span that 11+ inch gap.

Would a thin board of ceramic fiber board be strong enough? I'm inclined to keep using cast iron as it can handle many years of being knocked by wood.

I have a friend who built a double bell masonry heater... not really a batch box per say, but he splurged on the door and ordered a pretty custom glass/metal door from Canada somewhere.

Hey Daniel;   I have been experimenting this summer with different style roofs.
Here is one of my posts about changing out a cfb  roof,     https://permies.com/t/192389/roof-time

I was happy to hear about your success with cast iron griddle as that is what is on my 7"
As you see in my post the 6" got a 12x24x2" firebrick.
I tried Ra330 sheet metal, insulated with super wool and it warped in no time,
A friend in Canada is trying a kiln shelf this season, hoping it performs well.
Peter Berg has successfully used a specific brand of kiln shelf in the past as well.

I have tried ceramic board and it just can not be anywhere near wood.  
The stuff abrades in no time. Last year I was concerned my 6" roof could cave in from the weight of bricks on top.
As an insulating backer behind bricks or an RA330 sheet metal skin, CFB is great.
I will not use it in a firebox anymore.

Hey Matt;
It was probably a Pisla door.
You can see them at    http://www.northstoneheat.com/
These are the doors Matt Walker has recommended as a top-tier choice, or at least he did until he got his Dragon Tech door...  

Thomas, great thinking of the kiln shelf. I checked with my coworker who is a long time potter and she had 12"x24" shelves that have been pitted from the glaze she uses to give me for free. Excellent! I'll post a photo of the new 8" I'm working on next weekend.