Take another square with a mark on it even with the top (or bottom depending on how you look at it)
of the main pipe slot and mark it all the way around
Connect the dots with a flexible ruler ...
And whack it off with an axe! - or a saber saw - whichever is handier ...
Clean up the edges and add a handle and a gauge ...
...and a coat of BBQ paint ...
One Biscuit Bonnet ready to go ... or cinnamon rolls, or muffins, or homemade flatbread for "Q" sammiches, or new york pretzels to go with brats, or a steamer for rice/shrimp/veggies for jambalaya ...yada - yada - yada
You will also want a piece of expanded steel scrap cut in a circle just slightly smaller than the bonnet to set your biscuit pan on to hold it up off the disc.
Then start experimenting on where to set your propane burner to achieve 400 degrees in the bonnet. My biscuits call for 10 to 12 minutes at 400 - and I think I might try turning them to brown them evenly since there is no air circulation to brown the tops. Can't beat sausage/biscuits with eggs for breakfast!
, I knew you would come up with a good use for the valve covers!! 
....That should go straight to the pool room!
