Plain old box fan I bought at Walmart, attached with bailing twine and bungees and plugged into the cigarette lighter with an AC/DC adapter I got on sale at TSC (but they have them at AutoZone type places too). Not glamorous, but worked well.
"A good man will take care of his horses and dogs, not only while they are young, but also when they are old and past service." Plutarch
Someday soon -- the hay bale is a great idea -- it's so simple and obvious, I looked right past that possibility! Our huge, luscious orchard grass bales wouldn't work, but we can get a small bale of local grass hay that should work well, not be a pain to lift in and out, and would hardly be a loss if it got pooped and peed on.