If you bought a retail-boxed drive (model is suffixed with "G1R5" or "G2R5" depending on the generation) it'll come with an adapter in the box. If you bought a non-retail-boxed drive (suffixed with "G2C1" for example) you will either need to buy an adapter, or mount it with Velcro, rubber bands, etc.
On the other hand, the drive should be durable enough to simply place it somewhere loose in the case, unless you move the case regularly or plan to ship it.