I tried to look for something equivalent for those in-wall space heater units. They run on a different voltage, because of course they do.
I think the boxes were like $40, but they looked like electrical junction boxes. Clearly meant to be buried in a wall cavity.
Junction boxes are not meant to be buried in a wall. In fact, any junction box containing wire (not just connections/splices) is required to be accessible without removing any part of the building per the National Electrical Code.