Lines - strings, overpressure

I have a question about use of “overpressure lines” for string instruments. How can I make one line that begins as a “triangle”, to indicate gradual overpressure, is thick in the middle, and fades out as a triangle again. Or, just “two black triangles” together.
All these lines are available in the lines selection, but as three separate lines. When I attempt to put them together, they move easily out of place. Is there either one line that covers all three types, or is there a way of “grouping” the lines so that they don’t move around?
I have also tried to create a new line that has these qualities, but haven’t been successful so far.
Attached is a picture of what I’m trying to do, and another one of how it ends up looking like.
Thank you for the help!

If you can live with having, sort of, two wedges back to back, then you can achieve that by activating the Middle Thickness in the wedge editor. (see picture)

If you need 3 segments I’m pretty sure you need to have a straight line body and then use a pair of SVG graphics for start and end caps. Those caps will not resize with
the overall line length adjustments, so they would need to be drawn with that compromise in mind…