I don’t use Cast3m but I know that if we use Crimson editor we can make it a lot easier to create, edit.. or in general: to work, with Cast3m, with a more frieldly UI than Cast3m itself. (a totally MS-Dos’ son)
You can find Crimson Editor here, notice that its name has changed to Emerald Editor recently.
Also, the same with Notepad++ (free) or UltraEdit (Paid), you can use CrimsonEditor to work with many other programming language (C++, LaTEX..)
Have fun and good luck with your work!