6 Volt 50 Ford

I have a 1950 Ford witha 6 volt system.  I turned this car into a replica Police car. My question is, I would like to keep it 6 volts. I have two 12 volt lights one is a revolving light with four 12 volt bulbs mounted on the car and would like to use a seperate 12 volt battery to operate the 12 volt lights. I was thinking of using a 12 volt motorcycle battery due to the fact that the lights will not be on all the time. Only during parades and such. I was going to use a trickle charger to maintain the battery when not in use. Does anyone have any other idea's of with type 12 volt source I can use and how to hook something like this up.  Sorry for being so long winded.

Thanks,

Bob Sobers

This would work:

http://www.powerstream.com/dc6-12.htm