Don't build one. You cannot build something of equal quality to what you can buy. Building it yourself will cost nearly as much and be nowhere near as good. Why bother getting a flight cage for the budgie though? Just let it fly in your house. That's plenty of room for a little bird like that and doesn't cost anything EXCEPT
taking all necessary safety precautions and maintaining them always! You're better off spending your money on building a second front door as a safety catch than trying to build a cage.