I agree with earlier posters.
Provision of shade is key. Most buns prefer to be out of very hot midday sun anyway so will seek shade.
There is an animal sunblock available here (where pale animals really do risk sun damage to skin & eyes) for cats, dogs, & farm animals which I've used on those animals. However it is very thick & gloopy & does not say it is bunny safe. And as I said buns are really good shade seekers so it doesn't seem to be an issue.