I'll confess to no direct experience, but it's been discussed over on the Aeroelectric Connection quite a bit, and experts seem to agree that something as poor as the proverbial wet noodle hanging inside a main gear leg fairing will work fine for a marker beacon ant. Reason given is that the signal is so strong when you need it to work, the receiver will pick it up even with a very poor quality antenna. I've heard tales of people just tying a length of insulated wire to the center conductor of the coax.
FWIW...