I've seen two failures (nobody got hurt) during the vertical races at NSS conventions that involved chest boxes. In one, Darrell Tomer fell when the sides of his box abraded through his polyethylene slings. In the other, Bill Holmes broke his homemade chest plate at the 1990 convention. This is the plate. The plate is 6.6 mm thick (0.26 in). The upper break occurred in an area that was 13.1 mm. (0.51 in) high, and the lower break in an area 13.3 mm (0.56 in) high. The plate was made from scrap aluminum of unknown type. Failure occurred by fatigue.
Before you build your own gear, do your homework, and get the math, metallurgy and machining right.