The HG Iron-Blooded Orphans plastic model doesn't come with the eye effect that appears when the Alaya-Vijnana Limiter is released, so I made something similar using parts from the HGBC 1/144 Ninpulse Beams.
All you need are the parts and a pair of nippers, so it was relatively easy to make.
The ease of attaching it to the head will probably vary depending on the unit. (It might be easier to attach to units with large antennas, like the Barbatos series?)

Example of creating the eye effect when releasing the Alaya-Vijnana limiter | カメ's Gunpla works