I use a Xtra large cat litter box
Other suggestions I have seen are flat totes (as above)
And cement mixing pans from lowes etc
Like this , they have different sizes
https://www.lowes.com/pd/Creative-Plastic-Concepts-Large-Mixing-Tub-24-in-W-x-36-in-L-x-8-in-D-Drywall-Mud-Pan/1000642569