This should be fairly straightforward in LightBurn. I suggest building it up from primitive shapes and then welding all the shapes together. That will make the dimensions easy to manage.
If I was doing this I would create a couple of triangles move them apart stick a rectangle which over laps them in the middle and weld them all together.
I know lightburn has some nice drawing features, but when I need complex shapes I use Solidworks then export the drawing out and import into Lightburn. There are some very easy to learn free CAD packages out there.
The ‘O-Frame’ button is called ‘Rubber Band’ frame. It basically snaps a rubber-band around your file, and uses the laser to trace that shape. Since your shape has inward indents, the ‘rubber band’ around the object looks like this:
I’ve been a Corel Draw user since 1996, so when I started working on laser, all my drawings would be done in corel and then exported in SVG. However, as time has passed, I’ve found myself designing more and more directly on LB, and leaving Corel for only the most complex designs.