Even if the rest of this idea isn't chosen. I vote for "Armor of a Fat Knight"
I'm not sure what it should do, but it would probably be heavy since it would need to be big to fit a fat knight. Maybe it could give Guardsman a special bonus as well.
Do you think that while it is worn, you could reduce HP loss to the main tower? Like 10% +0.1/lvl.
And for guardsman, it could give either +1-2 multicrit or +25% crit chance.
Or you could go with the original idea of an extra inventory slot and being part of a set.