Just Be An Icefish
(I guess this is kind of a frame challenge?)
Icefish (Channichthyidae) can live water down to -2C / 28F. (Note that seawater freezes at a lower temperature than freshwater!)
They have simply evolved proteins that are adapted to operate at those temperatures, rather than at the higher temperatures preferred by... well, almost everything else, along with natural antifreeze molecules that keep their own bodily fluids liquid at below freezing temperatures. That indicates that the limits are a good bit below your stated 5C goal, and no technological solutions or additional exotic biology would be required to survive your merely-zero-degree environment.
But, say you want them to have a 5C body temperature anyway. It's a less extreme adaptation, and when the ambient temperature only gets a smidge below 0C anyway, 5C isn't that much of a delta to maintain. As PipperChip suggested, just adding more clothes probably would be sufficient after all, as long as they move around to generate muscular waste heat. But, you can also aim to have clothing that accumulates heat passively (or, for a lower-tech solution, evolved skin that does so). All you need are pigments that are black in the visible and near IR range (or whatever the peak spectral range of your world's star is, presuming that these ectothermic humanoids do not live on Earth), while having extremely low emissivity / high reflectivity in the far IR. Just as materials with the opposite properties (low visible emissivity, high IR emissivity) can passively cool themselves to several degrees below ambient temperature (by effectively radiatively coupling to deep space), your special cold-weather clothing would be able to raise its temperature several degrees above ambient by maximizing absorption of radiation and minimizing radiative losses.
Your humanoids could also do what a lot of cold-weather birds do to minimize heat loss: employ counter-current blood flow to the extremities, allowing them to operate at colder temperatures while conserving heat in the core. That would further reduce the amount of thermal energy that needs to be collected by clothing or skin, as well as maximizing usage of any internally-produced metabolic heat.