Insulation for bathtub cavity

What is the least toxic option to fill in the cavity in my cheap hollow bathtub. Spray foam is the norm, but I think about tomorrow's trash.
I'm 64, so really heavy materials would be challenging, as I am working alone. I'm already worried some will say cob. Any advice welcomed, tho.

Hi,
I think cob would be more of a heat battery than insulation in that context.

Perhaps perlite if you could find a good way to seal things up.

Some cellulose insulation isn't too bad.

Rockwool maybe, but you would need a frame or else get the blown in kind.

How about straw? That has been used for insulation for many years. As long as you can keep critters from living in it, and keep it dry, it would provide some insulation.