@Kaylim
I don't know if this is helpful, but I also have a few covers I'd commissioned with Tapas's square format in mind; this is how I turned one of them into a rectangular cover (not sure if this is final yet, might still play around with it since I feel like the text isn't visible enough):
If this can't work with yours though (certain illustrations don't make much sense anymore once cropped), I agree with @joannekwan 's suggestion; I'll probably also be using this method for covers that can't be zoomed in on.