On the box/prop you will need to change the orientation to match up with the figures body part. The part that is being constrained to is the leader and the body part the follower. The follower will change its orientation to match the Leaders.

Best way to do this is to bring in the box and place it under the hand. Constrain the box the the hand and turn the value up to 1. watch how the box twists as it gets nearer. Set the constraint back to zero and adjust the box in the same direction until it no longer twists when the constraint is turned up to 1. Now use those values on your other box and the body part should not distort.

One final thing is that you CANNOT constrain one body part to another body part on the same figure as that causes a feedback loop as the part tries to find the target which is moving and another cycle begins again.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13