Safety is all about odds.
How far one should go to ensure being seen by ships, depends on the conditions one are to sail in, and the density of ships. If crossing the English Channel were to be done frequently, I would most certainly fit an AIS, both receiver and transmitter - and a VHF-radio, of course.
As for passive radar reflectors, David Tyler warns a bit against putting up two of them side by side, because they may produce dead zones. In theory, this is right, but the dead zones are so narrow that it is unlikely that a small vessel, which yaws a bit back and forth, will stay invisible to a ship’s radar for long (for that reason, at least).
A number of years ago I read a test of radar reflectors in Practical Boat Owner. The most interesting thing was the ‘reference reflector’: This was simply aluminium foil (from the kitchen), stuffed in a big, black plastic garbage bag, which was then hoisted in the rig. This turned out to give a quite decent echo with no dead zones around it.
As a minimum setup for cruising in my waters, I would go for this:
(.. there are plenty of ferries in the area, but moderate swell to create sea-clutter, and there is only occasionally low visibility, mostly accompanied by light winds...)
I would bring a big bag made from sailcloth on board. The bag should hold about 100litre, be easy to close, and have a strong hoop at each end. In addition, I would keep a full roll of aluminium foil on board. These take up little room in the boat.
In the unlikely event of being caught in dense fog in a shipping lane, I would stuff the unrolled aluminium foil randomly into the bag, close the bag, and then hoist it in my spare halyard, at least half-way up the mast.
That would make us visible to both S-band and X-band radars. One must only hope that the radar watch-keeper up on the bridge is awake...
Arne