I have various things I do to keep my 2 cool. They are house rabbits and luckily the house stays cool with the fans on, but I also do the following:
I get a piece of kitchen roll tissue and run under cold water tap. Then squeeze out the excess water and gently wipe along their ears and over their face and then place at the back of their heads for a few seconds. They both love this. The hair on the back of their heads goes a bit punk rocker looking, but you can brush it back to normal later on.
I also have some of the fabric table dinner mats that i wet and place in my fridge for about 10 mins and then place in their pen and they love to lay on top of it. (You can use old tea towels for this too)
They have a concrete slab to lay on also as this stays cool.
Then the frozen water in a bottle, but i place this in a cotton pillowcase as they won't lay against it if it is uncovered and the cotton also stays cool especially if you spray it with a little water from a plant spray bottle on a mist setting.