Orthotics in ski boots?
Just go flat footed?
flat feet are not a life threatening disease :)
the solution is to begin by seeing a boot fitter that can tell the difference between a flat foot that has good arch flexibility, and a flat foot that lacks arch flexibility. from there the fitter can figure out which course of action to take in terms of underfoot support.
this is an advanced skill that most boot fitters do not possess. I only know a handful that can assess, and build a corresponding device.