OK - Lets bring in a bit of science. The effectiveness of a radar reflector if very much determined by the angle of orientation. This has seen particular high profile of late due to the sad incident of the Ouzo (link below).
The important issue is that radar reflectors are mounted in the "catch rain" position- any other orientation will severely com Lets think this through and we'll all see why.
A wave will always reflect from a flat surface at the same angle at which it arrived. Therefore if you place two reflectors at right angles, the wave will always go back directly to where it came from. See the image below. This is exactly what we want as it gives a big radar return to vessel looking for obstacles. Now given that we operate in a three dimensional world then we need three surfaces all at right angles) hence the shape of a traditional reflector.
If you mount the reflector with the vanes being either vertical or horizontal then this will give a strong reflection for radar coming from over head (e.g. an airplane) but will be naff all use for something loosely alongside you (e.g. another ship). In order to give a strong radar return to another ship then you MUST have it is the "catch rain" position (i.e. with the vanes at 45 degrees to either the horizontal and vertical)
Here ends the physics lesson