From a behavour point of view I would think it was best to have as little light as possible going through the mesh floor since bees are very sensitive to light levels
You've just given me an idea which will also solve another problem.
As the strongest light will be from the side rather than the ground, ditto the wind in winter, closing the open back side of the floor would help to keep out both of those.
I have some offcuts of heavy rubber sheet from a pond liner which will make an effective curtain for the whole of the rear side, hanging over the inspection board slot. That'll be much easier than trying to make a door to exactly fit the width of the slot.
The other problem solved is that it'll act as a baffle when administering oxalic vapour with the board in, the flexibility of the rubber allowing room for the power cable.
The rubber flap will also allow the board to be put say halfway in, if it's a particularly cold and stormy winter.
I've just made an additional floor and about to treat the wood, I'll see if I have any matt black or other dark paint to coat the inside below the OMF. That should also help stop light being reflected up into the brood box.
On a similar note, I think the bees get much less agitated when taking the cover off since I added transparent cover boards. I still keep the insulated covers on top of those and under the roof.
What I think is happening is that when the insulated board comes off first, the bees see the light, but they don't get any of the cooling or draft that comes from the hive being opened.
After a few seconds (a long time for a bee) when the transparent cover is lifted, they're not then getting any change in the light level, just an air flow/temperature change, it's not the normal transition from "hive closed" to "hive open", so doesn't provoke an as strong as normal instinctive response.
