In terms of stealth - not very. the plastic may help but at the end of the day you've got a big lump of metal behind you that works perfectly well as a reflector of radar waves.
The modern guns use a small doppler radar which is usually very accurate - they take a before and after picture of the area around and including the object and anything that has changed is deemed to be "moving".
The only thing I can think of is that the low height of the car makes it difficult for the officer to use properly. The biggest problem is that the guns operate on a very limited horizontal scan and really need to be mounted flat and level. The fact you said you were coming down hill suggests the officer would have to be tracking you coming down the hill which makes it more inaccurate coupled with the fact the gun will assume everything is level(ish) so you get a cosine effect. IE: the car is travelling down the hill or hypotenuse of a right angles triangle but at the bottom end is a radar gun that assume the car is on the shorter bottom of the triangle thus it underestimates the speed . Depending on how steep the hill this error can be really large (and this is before the error you may get from the operator trying to track a car downhill).
Quick table below has the accuracy depending on hill steepness.
99.6 5
98.5 10
96.6 15
94 20
90.6 25
86.6 30
81.9 35
76.6 40
70.7 45
Some of the ultra modern guns have this built in but the training manuals still state that the gun should be trained on a flat level car. Sounds like he was too near the base of the hill.
Count yourself lucky and go buy a lottery ticket
