Moving to RGSS3 Script Requests
Yeah, there are several scripts that will do this, so have a search (Master Script List might be a good starting point).
It's easy enough to call the save method via a script call, but you also have to prevent the player saving over the autosave slot (if you want to do that), but still make it available to load again.
If you don't mind them saving over it, then it'll be as easy as the script call whenever you want to autosave.
DataManager.save_game(0)
to save in save slot 0. You might have to try 1 if it doesn't like 0 (including if it lets you save with 0 but won't let you load it again)