Not all dangerous; netting not necessary; mesh floor is a good start but a travel screen would give peace of mind. Buy
travels screens or
traveltops or make them: info here at
The Apiarist.
Security will be assured by two ratchet straps
locked in parallel about 100mm in from each side of the box. Don't run the straps over the entrance because they'll interfere when you bung in the sponge.
Prevent the hives moving using luggage or straps or both. We're told that frames should follow the direction of travel to prevent frame slap when braking, but I've done it incorrectly many times without incident.
I'd choose a mild day, perhaps 8 or 9C, when the nest is likely to be loose. If you leave when it's 3 or 4C the tight nest will disperse in the warmer car, and if it's colder at the new location they'll be unable to move to reform the nest.