short and sweet is that it can't be done. believe me people around here have tried but it truly can't be done, RMXP's encryption has been cracked and that is that.
ARC on the other hand should be able to use a much stronger encryption and there has even bee some thoughts about providing special builds of the engine that are customized so that the encryption key is different.
but for now there is no truly secure way to encrypt you game, especially not with a script.
I believe there is a think called DRGSS that can encrypt games with a costume key but it require you to unpack the rgss dll use a hex editor to modify the key and repack the dll. then you should be able to use the new key to encrypt you game with DRGSS and distributing you modified dll with the game should allow it to not be decrypted unless they go searching for the key which would still visible in the dll.
but I've heard of no success stories where this actually worked. and the process is highly technical and no one is going to walk you through it or do it for you so.
besides even with the dll modifying I could still find the modified key and decrypt your game in a manner of minutes if I wanted to.
tl;dr
it can't be done.