Installation: Run the .mcr file once. It creates a polyOp Border Fill item in the polyOp category under Customize UI, which you can then add to a quadmenu or assign a shortcut to it (that’s what I’m showing in the preview gif). To use it, select a border vertex that defines the border corner and press the button/keyboard shortcut – this will fill a regular border. Or select two corners and press the shortcut, when filling irregular border. The border edge count has to be even.
Edit Poly support: If you don’t need Edit Poly modifier support, you don’t have to do anything else, if you want it, install the polyModOp function library. Version Requirement: 3ds max 9