The GARDENA gardening expert
Dear Ms. S., your fears are justified, as these light-dependent evergreens mustn't be put in a “darkroom” when protecting them from snow.
Such solid structures as you have described are basically only necessary where there is a need to stop larger masses of snow from breaking the branches off plants. It is generally sufficient to position the protective cover approximately 30 to 50 centimetres over the rhododendron, and above all to position it so that it stands at an angle.
This angle should be orientated to protect the plant against the windward side most hazardous to the plant (to counter snow and wind). In this way, the plant obtains more light and the snow is repelled sufficiently. It is perfectly alright for snow to lie on the leaves, as this represents a further, natural protection against evaporation. Another tip: Don't leave the rhododendron roots too long under the thick bark mulch layer (they are shallow root plants which require root ventilation), but rather clear this protection as early as possible (at the end of the worst of the frost periods)