What type of feeder are you using to feed the bees?
If it's a frame feeder, you may be able to get the syrup warm enough by putting it in the south position. Between thermal gain and bee heating interior of hive, you will likely get the syrup warm enough. Close entrance down to 3/4 inch so you draft minimal heat out.
Don't imagine you are getting a lot of flying at 5C. If temp gets warm enough to fly, there's numerous types of open feeders. The large rubber totes with a lid looked interesting. Put float material in such as wood sticks and straw and fill with syrup. Drill a number of 3-4 inch holes in front side of lid so bees can access syrup. Could just install a days supply of warm syrup before bees fly. Could take tote inside in the evening to keep syrup warm and once you know daily consumption just add that amount of warm syrup each day. You don't have to move in doors each evening that way. Put tote in a sunny location out of the wind.
There was a thread about a month ago.
Other options are 5 gallon pail feeders put out each day, barrel feeders, V pig trough style, and I'm sure there are more.