So, I was
also intrigued with the idea of using a zener but not sure how to go about
that.
The radio appears to draw a max of 100 ma @ 6 vdc.

Jack-

As Chris pointed out, the radio should still function when the four C cells are
down to about 5 volts. Since fresh C cells are about 1.55V, I would go for a
6.2 volt zener diode. Remember, it is acting as a limiter rather than an
absolute regulator.

Series resistance is chosen for (5.8V - 5V)/0.1A = 8 Ohms. Maximum current in
the resistor occurs with input of 8.5V, I = (8.5-6.2)/8 = 0.2875A. Maximum
power is P = ((8.5-6.2)^2)/8 = 0.66125W. I would choose an 8.2 Ohm 2 watt
minimum resistor, preferably wire-wound rather than carbon. (Carbon has poor
aging characteristics.)

Zener power is maximum when the radio is off. P = 6.2V * 0.2875A = 1.7825W. I
would look for a three to five watt 6.2 volt zener diode with wire leads. If
you want to heat-sink it, I believe there is a ten watt stud-mounted "R"
(anode-to-stud) diode available if you search for it. Otherwise you would have
to insulate the stud of the standard cathode-to-stud configuration.

Fred