I'd like to make a semitransparent shader with the principled shader in mantra that looks like the one above, which was made in C4D/Arnold. I've been trying everything I could think of but I just can't seem to get it right! Anyone have any ideas?

Anyone? Come on, I know I've seen this done with Houdini before!

What have you tried that didn't work? It just looks like transmission with maybe variation of transmission color based on local thickness, or perhaps just a painted map. The real deal would have absorption and scattering varying volumetrically, but I don't know of any renderers that can do this, aside from writing your own raymarch (which is doable in mantra). The source image just looks like a surface varying(not physically correct) approach.

Then what would a viable method in Mantra be?