removing all the boxes is a good start.
if you have any stickies put them in and if there is anything salvageable in the hive wack it in the freezer for at least 36 hours.
You said your hives are in the sun. good start, now block all vents to the hive (strong hives need high brood temp ~38'c and high brood temp bees live longer than 34'c brood. twice as long actually) & reduce your entrance to almost half width.
Don't worry about over heating of the hive, the bees can handle it until the outside temp reaches 40+'c them you will need to open the vents.
Remember swarms don't have the luxury of vents in trees, so they are not needed.
if you have brood available to you, place 2x frames of brood in between your stickies, make sure it is capped, and no need to transfer bees with it, keep your old bees from the slimed hive. If you have a cage, cage your queen and place her between the 2 frames of brood, as the new bees hatch, they will look after the queen, feed her and get her laying again.
there are other tricks to kick starting your slimed hives also,
pollen patties will kick off brood production again
pure icing sugar (not icing mix) on top of the top bars for some food to kick nectar production.
you can put icing mix in beetle traps (it dehydrates them once eaten, and they love the sweetness)
you can make a sugar and cinnamon liquid mix and spray all your frame with it, it will get the bees cleaning the frames & cinnamon is antibacterial.