Any Small Mask Files Out There for Practice?

I’d like to simulate data in R using Neurosim. I am having trouble making the resulting images look like brains. In the 2D case, for example, I get a rectangle with blobs of activation. Looks nothing like a brain though. I’m assuming it’s because I don’t have a mask applied to the function. All of the masks I’ve seen in practice datasets are too large for me. I was wondering if there are any masks out there that would make my data look like a real brain, without being larger than ~50 voxels in each dimension. Thanks!