I bought 2 of these mirrors, for my wifes and my BMW's after driving a rental
Golf on the Autobahn and appreciating them.
They most certainly do work and give you more confidence when lane changing.
I bought them from Auto Teile Unger which is a chain around Germany, for
about 18 Euros ea.     They glue on the original glass with some heavy duty
double sided tape.
I didnt see one specifically for our vans though but they may have something
that could do a reasonable job of fitting.

cheers,  Steve