It looks like you can get reasonable SMD boost chips and pack them and their entire support infra in <10mm https://hhtronik.com/product/uboost/ https://hhtroni

It looks like you can get reasonable SMD boost chips and pack them and their entire support infra in <10mm https://hhtronik.com/product/uboost/ https://hhtronik.com/product/uboost/ which is about the same as two batteries as used in this project.