We pretty much do what donnamt does, except our journey is only ever 2.5 hours so we don't take stops, even if you do stop for a few mins they are a bit too anxious to be responsive enough to eat....if we did longer journeys we would stop as donnamt does for longer periods to allow them to 'come round' as it were.
We also use a drop or two each of Bach Five Flower rescue remedy on a raisin each before we set off, I find that helps loads personally.
We cover the carry cage with a towel leaving gaps for air to flow through but keeping it dark enough and so that they can't see the countryside whizzing by. I also keep the car at a good cool but not too cold temp....cool is good, stressed buns get hot, but motionless buns lose heat quickly too. We use air con set to about 19 or 20, less if the sun is out.
I give them both a big plate of freshly washed and wet veggies/leaves before we set off and pack them up into the carry cage as the last thing we do literally as we are going out of the door. We take all their existing blankets etc with fur and scent on them, a used litter tray and all their current toys so everything smells familiar. We also leave them to relax in peace once we have arrived and set the pen and lino up - just making sure that both are tucking into fresh hay and grass in their trays as soon as they are in the pen. I take my medicine kit and the no.s for a local vet which I have looked up before going - including emergency vet no for out of hours. The RCVS website will hold a list for each area. Good luck, as donnamt says, us humans get more stressed!
I find by the time the buns are settling in to the surroundings....it's time to come home again! :lol: Try and stick to normal times and routine too and I would suggest travelling them when they are naturally sleepy if you can.
p.s lift any towel over the top off every now and then for air change - heat rises and it can trap in a lot of heat.