Thank you for the welcome! I've been using and loving Paint.NET for years, but only now am joining the forum 🙂
I'm making some how-to photos. First I do some minor image editing, then on the last step I draw a box around the target to help highlight it. For visual consistency, I'm using the same color across all photos. Unfortunately, the primary box color is similar to the background color for some of these photos, so the box needs a second color around its border to increase contrast. In PowerPoint this is easily accomplished and repeated with the "Glow" shape effect.
To accomplish something similar with the Paint.NET Custom Shapes, I wanted to extend your "Box Emphasis" custom shape. A similar glow effect can be had with setting fill mode to "Draw Filled Shape With Outline", but unfortunately the Box Emphasis shape does not scale well. If I want to make a tall box, then the top/bottom edges become much wider than the left/right edges, and vice versa for a wide box. To fix this, I wanted to add a new "Frame Width" parameter which would programmatically set the width of the edges so they're consistent for taller and wider rectangles.
"Glow" Shape Effect in PowerPoint, with base image from https://commons.wikimedia.org/wiki/File:Smiljan_Memorial_Center_Modified.jpg