[mmgy]

If you've got the crate tethered securely up behind the cab, there's not actually that much wind there. Instead of trying to cover the crate with a blanket that will buffett about in any breeze, how about plywood? Cut a sheet to fit over the top for sun protection. And cut sheets to secure to one or two sides if you like.

I don't like seeing dogs in the backs of trucks, no matter how secure they are, but since this is a temporary situation for you, I think you'd be okay.