Axle locked in blockhead - how to transport front wheels?

I'm getting ready to mount blockheads on my truck tool box for the purpose of transporting our bikes on vacation later this summer. So I'm curious to know what others with similar transport arrangements might be doing with the front wheels of their bikes?

I suppose I could fabricate something out of 2x lumber and plywood to haul them upright (as opposed to rattling around in the bed of the truck), but I've not seen anything else around town.

Ideas, or pictures even?

Thanks in advance,

Rick