The Flow 2 uses a large, hive-wide entrance. This encourages robbing and is drafty.
The entrance is 32m in height, I believe, but need only be around 10mm. This makes an additional entrance screen possible, without sacrificing ventilation.
To reduce the entrance, fit a 32mm-thick piece into the entrance entirely. Along a 150mm span, route 10mm at the bottom, to a depth of 22mm. From the back, route at 10mm from the bottom, at a depth of 22mm.
The bees may now enter from the front, travel forward to a back wall 22mm in, with a vertical shaft 12mm wide above. The wall is 10mm high, and the top of the shaft returns to a 10mm tunnel opening into the hive. Bees can ventilate here, but it’s hard for mice or water to get in.
Route off 1mm from the top, to a depth of no more than 20mm from the inside. A second screened bottom board rests on this shelf, just underneath the frames. This bottom board extends to 25mm from the back of the hive—potentially resting on rails along the sides. Use a #8 hardware cloth for this.
This screen leaves 32mm for the bees; an alternate design would bring the entrance all the way to the top, raise the wall to around 12mm, and run the lower bottom board from there, giving only 12mm clearance.
In either case, yellow jackets and other robbers must enter the front of the hive, climb up into this passage, and proceed to the rear. This will not end well for the robbers.
This is ideal with a Warre or AZ brood, although operating with Langstroth frames is also fine.