In the look and feel brief, it says Highguard typically uses Kite and Heater shields but the only ones I can find have straps instead of handles. So I’m wondering would it be acceptable to hold the straps so that the shield would still be taking up one of my hands whilst being wielded?
Example
It looks like what you have there is a nice shield that could be worn on either hand: left or right, through one strap and hold the other.
(What manufacturer, by the way?)
I see your confusion on this, re the line on shields from the wiki:
“A shield must be gripped by a handle held in the hand to protect the wearer. A shield that is simply strapped to the body does not protect the wearer. All blows that strike a slung shield are considered to hit the character wearing it.”
While this is something to go to the weapons checkers with, I suspect that holding the one of the straps AS a handle would be fine.
Also there are many vendors with many shields for sale on site. Its where i got mine last event
Aah, I see thank you! This shield is one of the Epic Armouries ‘Ready For Battle’ line. There’s a few other shields I’m looking at/debating between but this one was the clearest shot of the straps.
Ive definitely seen centre grip Highborn shields, we ordered some from Eldritch in the past for our group 
Yes, for those heater shields you pass your arm through one strap and grip the other as a handle. This is what we see on many historical examples, so you’re in good company!
