A Firdell "Blipper".
Being a sail training skipper responsible for young people on board, this one works best.
It is also recomended for coded commercial vessels.
Despite its size it is light and will not be a problem for your size of vessel.
An addition may be a radar transponder to make your yacht larger to others.
"Practical Sailor" (a "Which ?"-like magazine for sailboat owners) made tests and concluded that the octahedral reflector had the best returns, but had "holes" between the reflected lobes. I found out that I had the best results using two reflectors, one "Davis Echomater" hanging from the port crosstree (about 15 ft above deck) and another, smaller HYE reflctor fastened to the starboard side of the mast, close to the mashead (about 30 ft above deck). Since ships' radars cannot distinguish two reflectors 15 ft apart, they show them as one, bigger echo. Various ships I talked to offshore have reported seeing my boat at 10+ moiles away. One captain said: "You have a very good echo; not a ship's echo, mind you, but a very good echo".
john
I remember anchoring off Flatholm Island in the Bristol Channel where we explored the island and spoke with the then lighthouse keepers,who had tracked my boat (then a hurley 22 with an aluminium mast and my friend in a Yachting Monthly Eventide with a wooden mast. They had picked me up quite a way off just from the metal mast of my boat, but had no radar image of my friends wooden boat and wooden mast.
I also remember a trip at night around Lands End in the late 1980s on the same friends boat then a Colvic 26 when a spherical plastic radar reflector he had bought which was hoisted to the crosstrees descended to my head very rapidly when the string broke,and consequently split open and was found to contain no more than crumpled cooking foil.
I now have a Firdell Blipper but have not as yet been able to prove or disprove it's effectivness.
Best is an active transponder, but most expensive.
Next best a lens-type, 1st - Cyclops, 2nd - Visiball.
Best of static metal types (according to test statistics) - 1st - Echomax, 2nd - Blipper.
I have tested them all in the past but don't know if the lens-types are still made.