Add an inch or two of dry, clean builder's sand to that tub and it might be even better. The sand adds weight so it is less likely to dislodged with a bump. The sand helps keep stoves upright. If you want a flat hard surface add a 12" square ceramic tile on top of the bed of sand. If you spill fuel sand adsorbs it instead of it splashing. If you knock the tub over the fuel dampened sand lays in a pile instead of the puddle running downhill, usually in the least convenient direction. If/when the tub develops a leak sand helps keep the liquid fuel in.

Builder's sand is cheap. Most building sites that use masonry have a pile they are often happy to get rid of once the masonry is done. A five gallon bucket full would do it. If possible avoid beach sand as it contains salts that can cause issues.

A second bucketful would be a good second line of fire control after your wet towel. A metal lid sized to cover the top of the tub would be good. The worse conflagration imaginable in your tub could be under control in seconds when you drop cover it. Once covered you can go inside for a nap and return when convenient.