Hi ;
You folks have heard from me before on the benefits of LEGO and LEGO Wannabes ! Now say you want create a mold for a part for a Ship , Car , Tank or Plane .
Make your box from LEGOS or their Wannabe Competitors products . Super Glue them together and then that way they'll be there for the next time . Sand them inside if you want with 320 grit then 400 grit and they will release the mold you made very easily .
Using this method is cheap easy , and always do-able . Also , using the narrow plates 1x4 , 1x6 etc. you can imbed them in the resin or mold material for locating pins for a two part mold !