The manufacturer may have instructions but the NEC is silent on the mounting. Box fill is 2 conductors allowance, same as any device. That doesn't mean it will fit.
Common sense says use a box that has enough room and mount this securely. Twist lock implies there will be some extra strain on the cord and you won't simply pull out the plug cap. We used a lot of twist lock and usually put them in a 4" box with 4 screws per device.
