Earth Oven Form

Constructing an earth oven generally requires a form that the mud rests against as the oven goes up. from Earth Oven Adventure
Most folks use a simple sand mold.

from The Bread Ovens Of Quebec
But traditionally in Quebec, a framework of alder saplings was used.

Here, a boat builder has used his skills and the materials he had on hand to build a simple framework of cedar, plywood and cardboard.

And then shares a pizza he made just a few hours after the oven was constructed.

Hot Pants

Baking superlative breads requires steam, and one way of adding steam to the oven is by sealing the oven door with with something wet. My son graciously donated a pair of worn out pants, and I soaked the pants in water and mushed them around the door after the bread came off the peel.
Spraying the oven interior also helps, but in this case, I did both.

Get It Hot

Some folks who have recently completed their earth ovens are often a little discouraged because they really don't know how to bake in them. You need to practice, and you definitely need to get your oven hot, really hot for pizzas.

How hot? The oven floor where you'll be baking should be about 650 degrees. How will you know the temperature? Well, easiest way is with an infrared thermometer with laser. I bought a Mastercool from Amazon for about $35.00, and they even give you a case and a dial thermometer.
Of course, there is the low-tech way. You could toss some flour on the floor and observe how long it takes to turn brown, or ask the person who you live with to reach into the oven and calculate how many seconds it takes before you hear a scream.
If your oven is not hot and fiery, your pizzas will not bake properly.