go to a steel supplier with a plasma cutter buy a 4’x8′ sheet around a 1/4 inch thick get them to slightly contour in power roller get them cut into 10 x12 inch rectangles with corners cut 45 at bottom in inch in 60 degree cut both top corners one inch in gives better contour for arm movement.
some steel wholesalers have cuts that can be bought for cheaper as the previous person paid for the bulk of cost rest is scrap. only thing is the thickness can vary would not go under 1/4 as 5mm is min thickness to stop a green tip 5.54 i go with 5/16 plate myself that stops a 30 cal cold multi hits though the kinetic impact will leave you feeling like mule kicked you.
you can get it coated with rubberized bed liner(truck stuff double coat) to help with spalling from impacts. way cheaper than store bought and you get.
the store bought ones last as long but are about 2 times more expensive…