For a while I have been looking for a way to create custom alerts or modals but then XSAlert came through for me. The app is really nice and very easy to use.
This is actually a great little tool if you're a die-hard Twitter creator. The tool, especially with the premium features (with a one-time donation) really help your Tweets stand out. It's like the Pablo app from Buffer specifically designed for Twitter. Download was simple and there was more ability to customize font and image placement compared to other apps that attempt to do the same thing.