Fiberglass Reinforced Plastic.

Would not a gallon milk jug actually suffice?

In my area there are several plastic suppliers who carry a wide range of materials in many sizes and thicknesses and sell to homeowners. Try googling this....

" And you don't start with a circle to make a cone. " Nonsense.

Making a Cone

I have flexible plastic cutting boards that sound about right. It would have to be fastened together somehow, but duct tape exists.

Look at BBQ grill mats. We bought some that are larger than plastic cutting boards.

Quilting template plastic, available at quilt stores, fabric stores, craft stores, Amazon Amazon.

• It must be flexible enough to be cut and bent into a conical shape, somewhat like a funnel,

How about a funnel?

My thought was an Elizabethan dog collar, the type that a dog would wear after surgery. They come in different sizes. check at a local vet office, theyre not very expensive and are already to be cone shaped.

https://www.petsmart.com/dog/health-and-wellness/first-aid-and-recovery/dog-mxandtrade-plastic-e-collar-5319506.html?gclsrc=aw.ds&gclid=EAIaIQobChMIvsfC49bH_AIVldzICh1cEgGrEAQYAyABEgKKrfD_BwE

Elizabethan Collar can not be closed at the narrow end.

If you get the kind from the vet's office, you can make it into a cone with a closed, pointed top, or you can get creative with the type from PetSmart and put something like a pie tin on top.

Nope.

Example

Stax, i'm actually quite familiar with e-collars. The open top might even work better for the OP, who mentions that the water tank is cylindrical.