Starting a fire? Wow! Maybe tha composting is more of an option than I thought; I'll come back to it.
My current waterer is a 3 quart, vacuum sealed ice bucket, 7" outside diameter, 6" inside diameter, 7" outside height. It is set on cement blocks to raise it 8" above the floor. That puts the bottom of the bucket a little below the top of the bedding.
Alternative waterer is black rubber dish very roughly the same diameter and shorter. It should work the same in the following with a few tweaks; I'll work through those tweaks later.
If I replace the cement blocks with a water reservoir and surround both with insulation...
Uline plastic pail options are 1, 2, 3.5, 5, 6, and 7 gallons
1 gallon is 7.65" outside diameter at the top, 7.04" inside diameter at the top, 6.20" OD at the bottom, and 7.59" tall without a lid. Too small.
2 gallon is 10" at the top, 8.36" at the bottom, 9.25" tall without a lid.
The rest are 11-12" diameter at the top, 10.42" at the bottom - big enough to nest the ice bucket inside with some insulation around it.
3.5 gallon is 11" high w/o lid
5 gallon is 14.5" high w/o lid
6 gallon is 16.75" high w/o lid
7 gallon is 19.62" high w/o lid
6 gallon looks promising - nesting the ice bucket (or rubber dish) so the rims are about even leaves about 10" for a water reservoir of about 3 gallons (more for the rubber dish) And will get the top of the waterer to about what it is now... which works well.
If I sacrifice two 6 gallon pails from other purposes, I can switch the pails instead of containers inside a bucket... possibly easier, certainly the most capacity. This option would need something to rest the ice bucket on. Also, it would need insulation between the sides of ice bucket and sides of the pails. Also, it would need insulation around the outside of the pail at least below the level of the ice bucket.
To rest the ice bucket on: Pvc legs? That would need something to hold them in place, maybe something to separate the insulation around the ice bucket from the reservoir. A partial lid would help.
So far, it seems promising.