Hi, we have thousands of hours of product design and manufacturing experience! From just what we've read, we think that a nichrome wire embedded in a flame resistant sleeving and attached to the foam wedge, covered in a soft cloth case would be perfect for your needs. It would be able to keep your pizza warm, but you shouldnt expose foam to pizza cooking temperatures (not sure if that's what you meant! We're pretty it wasn't but we just wanted to say that just in case) :). Nichrome wire is what is used in most heating pads, so we think that design would be perfect for what your looking for.
Even though you haven't accepted my bid, we'll at least start calling suppliers and seeing where we can get foam in the dimensions you're requesting. Also, when you go to produce these, we can cut the foam to spec, and create circuitry; as well as 3D printing a housing for it. We're an all in one solution for your needs! We hope you'll choose us, thank you and have a great weekend!
EDIT: Just an update, we have a foam supplier that can supply real Memory foam in the size provided for, $37.50 a wedge. Additionally the control circuitry for the entire thing will cost ~$20-30 (this is for components, PCB, casing, and for us to assemble them for you), then miscellaneous things like fabric and threading for the clothe components. Have a great weekend!