Mike, you have the UUNI steel though right? The one I want is thick, made by bakingsteel.com, they got a few requests from UUNI owners to make them to the exact size of the stone to fit the UUNI so I e-mailed them about it and am waiting for a response. I imagine if demand continues, they'll offer it online without having to have it custom ordered. It's going to be expensive, those baking steels aren't cheap. I have 2 of Broil King's "grilling steels, one is round and one is square. Stephanie makes smash burgers on them on the keg, they work great for that but I've always heard that they are much better than stones for heat retention so we shall see!
Here's what the steel looks like in the UUNI
There are other mods that people are doing to it like a flame shield to protect the edge of pizza from getting burned (this particular person is using a VERY THICK 3/4" pizza stone trimmed to fit the UUNI)
This guy just uses aluminum foil as a flame barrier
And this UUNI owner decided to mount a thermometer in the door
I doubt I'll ever worry about the door thermometer but the steel and the flame shield/barrier ideas I like!