Good advice above. Fans are great, but remember if you don't cool the air first that all they do is waft the warm air around.
You can also insulate the roof of your rabbit's home so that the sun doesn't warm it up too much. This can be done simply by making sure that it's covered by something that doesn't absorb the heat.
When you say your buns are going outside again - is that completely outside or into a shed or the like? Keeping the hutch away from reflective surfaces will also help. So if it's in a shed then make sure it's not where the sun can shine through a window and create a greenhouse effect.
One word of warning about ice bottles is that this will raise the humidity of the air in the hutch - so be aware of this.
Probably obvious info - but worth mentioning, is to make sure that the hutch is in a shady area, and not where the sun can beat down full pelt.
Good luck - I'm sure you'll do just fine.
Kind regards,
Jacki - Small Pet Select