To craft this in the cheapest way possible, use scrap metal ingots ( easy to make a farm for and harvest with a delicate harvest theri, you should be able to make a simple farm and you probably already got a theri, it is also easy to get in case you don't have one. Scrap metal is a 1:1 instead of a 2:1 conversion rate, also cooks alongside normal metal iirc ) and 380 HARD polymer, for the 620 left, you can use organic polymer or corrupted nodules.