Makes sure their in the most shaded part of your garden.
Freeze pop bottles of water and put them in their bed box's to keep it cool.
Hang wet sheets over their hutch's, sheds,run etc.
Trays of water to for them to paddle in, and to drink from so they dont dehydrate with just bottles (especially if fed dry).
You can buy them ice pods from petsathome also which are like freezer packs.