Thanks Alex, for the file, I will close the upload link.
I got three warnings about missing plug ins. 2 material and cycles. So I might not see what you see.
Yes, that the one: The Hair option is under Hair> Attribute Manager> Guides> Type: Square.
I checked and perhaps the Shader Backlight (Luminance channel, extra material) might be fast enough for an animation. Subsurface Scattering for a still is cool, but way too slow.
The Hair material will provide the Thickness and other shape informations if needed.
(However, you might switch away from Soft Shadow. Perhaps over the past decade+ I have enough complained about, that I don’t have to start here over, it is just a fake.)
All the best