My first crack at making a core casting template worked alright but I ran into issues with alignment when putting the core into my mold impression and putting the flask back together.   So I decided to make a flask just for my ring cores and added alignment aids to reduce issues.   

I designed this with SLA printing in mind so some considerations may need to be made to ensure this prints accurately with FDM.    When making core molds you may want to adjust the sizes to account for tolerances (by default I've added .20 mm to the interior diameter of the core mold but not the core placement and this works fine for my printer.)

If you are new to casting I recommend using a cheap aluminum flask until you can confidently control your torch and pour metal without spilling as this is plastic and will melt or burn.   But since it's only plastic and printed quickly you can always print another part if it's burnt.  

Feel free to remix this or even use it however you please.   I feel like there are a lot of expensive tools in being a jeweler and if this helps someone new to the craft get started I'll be happy.    Bonus points if you bang this out from a block of aluminum or something flashy.