Hi Tom, in my experience robber bees can smell honey through the finest of cracks. Therefore you need to eliminate any cracks the bees can smell honey through.
I would switch to a solid floor, I would temporarily block the vents in your lid, then I would suggest to wrap masking tape around where the boxes meet with each other, as well as where they meet the lid & bottom board. If you don’t have a strong population, you can reduce the entrance to about 2" (50mm), assuming it is 3/8" (10mm) high.
Work on 5 sq.cm. Therefore if your entrance is deeper that 10mm (1cm), you can reduce it to less than 2" (5cm).
On top of that, use the towel & make the robbing screen as @Eva suggested.