I would not recommend using natural gas for trying to boil crawfish. Natural gas does not burn nearly as hot as propane. Propane has almost 5 times the heating value vs. natural gas. Natural gas takes about twice as long just to get a pot full of water boiling.
Get a burner similar to this kind that pre-mixes air with the fuel. With the price of propane you will probably save enough money in 3 or 4 bottles of fuel to cover the extra cost of a good burner.
This type burner is a lot more efficient than a burner that just has a straight pipe. The straight pipe type burner does not pre-mix the fuel with air and this type setup is very loud.
SUPER COOKER BURNER ONLY 11" 60,000BTU