Couple questions, are they baseboard radiators or cast iron? There's things you can place on cast iron ones that hold water and evaporate as needed.

I use these natural stone (terra cotta?) Ones that absorb the water then evaporate as needed. They come in a pack of 2 on amazon- bluestone I believe is the brand. We got a few and have the larger one in the dryer rooms and the smaller ones in the rooms that need less help. Have to maybe fill the bowl once a week or so during the winter.