Use mojombo's Clippy which can be downloaded from GitHub.
How to create a custom button
- First create your button and save it as a PNG file.
- Change library.xml to use your button.
- Customize the code behind the button by modifying clippy.hx. You can, for example, remove the "Copy to clipboard" label.
- Set the width and height of the button. You have to set it in almost all files (compile.hxml, library.xml, etc).
- Compile the SWF file:
swfmill simple library.xml library.swf && haxe compile.hxml
There are no good alternatives that I know of. I tried ZeroClipboard and other solutions, but they were full of bugs and not customizable.