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.

Can we seea photo of said bath to get an idea please?

The last time couple of times I did this, I used trash that was already in existence...stuffed in all kinds of clean plastic, wads of bubble wrap, styrofoam peanuts, etc. until the cavity was full.  Now we bag all this kind of stuff and send it to our attic, which is inadequately insulated.  I've been taking old carpets up there too, and have considered cardboard but am vacillating on how much fire hazard I'm game to risk. Thanks to this kind of "recycling by sequestration", we've been able to cancel our garbage service completely.  Only nasty food-soiled plastic has to be compacted and contributed to sundry garbage cans in the wild....everything else ends up either at the recycling center, in the compost, the woodstove, for mulch, buried in planting holes, or sequestered for insulation.

I'm with Alder Burns. I used plastic grocery store bags to fill spaces tightly to insulate them. I couldn't get anything bigger through the holes I was accessing with, or I would have mixed in things like chunks of styrofoam packing mateial etc. Pack it tight with whatever non-rotting stuff you can find. If you need more, check the trash cans behind furniture stores, LOTS of plastic bag type material in them.

My desire was to support the bottom and dull the empty sound and feel. So, I am thinking the plastic bags and the collection of Styrofoam inside the narrow parts and a traditional sand base on the floor portion for some heat retention. webpage bathtub homedepot
This is the tub I bought.. steel but hollow
Anyway, thanks Alder & Isabel for reaching out and for the advice.