If you really want to build one this book would be worth your time.
When I think of mud I see clay.
I wonder if fire bricks would make a floor that is smooth enough.
Depending on the kind of peel you use, the brick edges could prove to
be impediments to sliding the peel under the crust. For that reason I
am thinking of setting in a pizza stone or some other fired tile. I
will no longer buy the round pizza stones; they break at any real
temperature. The square one I use has been good up to 600 degrees so
far. Before I would use it as a floor on a wood-fired oven I would
check with the manufacturer to make sure it could handle very high heat.