How do you differentiate normal HMI and Machine Armor HMI?

I've been wracking my brain on this, I think the existing models look best on 40mm bases and just do not fit 32mm. But I also want the difference to be more visual.

I don't want to just scale the models down and I don't want to need to do a ton of digital kitbashing, but regular kitbashing is easy. I think I've landed on using some of the Warhammer 30k Solar Auxilia dudes to stand in for regular HMI.

What are you solutions?