Just buy one. Unless you live in California or Maryland, they are not controlled. Flamethrowers are used in agriculture, for sugarcane harvesting among other uses.
(An AR-15 is probably much more useful for home defense, though.)
You can make one pretty easy. :)
“Flamethrowers are used in agriculture, for sugarcane harvesting among other uses.”
Harbor Freight sells a pretty good imitation for about $30.
Connect it the propane tank from a gas grill and it will burn everything that gets within 10 feet.
Instantly light brush piles or a wood burner.