Backfill around a sewage basin?

In my spare time I am stubbing in a basement bath. I started by digging the hole for a sewage basin and found the soil to be clay. We do have a seasonal high water table so I bought the Zoeller basin with the anti-float flange on the bottom.

I tend to over-think things, but I don't feel that I will be able to adequately compact the native clay around the basin without making the slab cut bigger than I'd prefer to tamp around the basin. Obviously, washing loose clay in with water would be a real mess. I'm thinking about using a mix of sharp sand, clay, and pea gravel, but I'm interested in hearing what the pro's think.

basin

Wash sand in with a hose. The basin is not going to go anywhere. Just do not use so much water that the basin starts to float, (fill it with water first to prevent it from happening).