I've got a similar problem - just haven't got round to sorting it out. I chose breeze blocks as they are cheap, big and light, compared to bricks. It also only needs to be 2 high, so will be more stable than smaller blocks. My idea was to build low walls (just by stacking the blocks with a half block overlap) underneath the hutch at about 1m intervals from the front to the back of it. You could also make just a pillar at each corner and some key points along the length. Otherwise, something like railway sleepers woud be about the right height. You can usually get short lengths from wood yards. Use them like floor joists.
Usually, I just use whatever I have lying around - often bricks, but I wasn't happy just stacking them with a very heavy hutch on top (mine is a bit higher than yours). Stacks of smaller paving slabs at each end and the middle may also work. They don't absolutely have to be the exact height, as long as there isn't more than a finger or two gap - if they are there in case the legs give way. Just think about how it would fall in that case, and so they won't trap any bunny feet etc, if they normally go under the hutch.