You could source disks of foam from a local foam store (I actually have one down the street) and then rig up a hot-wire to cut out the bottom. I imagine you'd determine the curvature of the "radius" from the center to lip, then somehow find a way to keep the form while connecting a wire of some sort to a battery to generate enough heat for it to melt through the foam. I've seen them use a machine like this on a home decoration show, to cut out shapes and forms for pillows and stuff.
Sounds kinda fun actually 😛