I think looking at how the power companies set up their cross arms would be a good idea. They've solved the issue of dealing with winds (most of the time). A wooden cross arm would work, but you'd want some bracing to hold it level. I'd pay close attention to making sure the wood used won't decay/dry out and fail.
If it was me, I'd probably make two cross arms out of metal. Two 10 foot lengths of Unistrut properly braced would be a good place to start. You can use pipe clamps on the unistrut to hold some vertical pipe to mount the antennas on. U-bolts would work, also.
You can get Unistrut and the associated hardware at Home Depot. If you go there and start looking at all the various hardware, your imagination will take over and you'll find a good solution.