At a range of 5 feet the detection cone on an E4 is almost 3.5 feet wide.
I'd imagine after a couple of feet it will see the superstructure of your robot before it sees the 1 sq ft lexan target.
Possibly, angling the sensor to catch the arm as far from the superstructure as possible might help.
It'll probably see the bottom of any tube you're carrying, so take that into account.
Look at the
comparison chart on page 5 to see the relative shapes of the detection cones.