The Roman shield was held by a crossbar across a hole in the center of the shield covered by the metal shield boss. The 2001 K&C Romans had straps through which the figures arm fitted; like a Hoplite shield, this was incorrect.
The Testudo formation was used primarily in sieges to approach fortifications and defend against stones and arrows. Placing the hands above the shield was risking injury, also using both hands to support the shield left the Legionary unarmed if an adversary sprang onto or into the Testudo formation.