I would think you could go as high or low tech as you need.

Low tech - some velcro pads on both sides of the hinged edge - near the top and bottom.. then have paint stir sticks with the corresponding opposite velcro pads.. when the shield is 'open' stick on the sticks and that should keep it from flopping close.

Alternatively you could look at latch-bolt options to sit a few places over the hinge part to lock things into place.

https://www.acehardware.com/departments/hardware/specialty-hardware/barrel-bolts/5368725

https://www.amazon.com/National-Hardware-N102-855-Swivel-Staple/dp/B000BQO4YQ/

https://www.amazon.com/Self-Closing-Latch-Spring-Door-Black/dp/B07P7FGZ1D