I have listed two methods for building a Varroa floor.
One is as below and the other, please download this file 'Building open mesh floors by Graham White'

Top		Three pieces 21 x 21 mm for the top and halved jointed as shown
Mesh		suitable size of mesh to fit your size of hive
Base		Three pieces 21 x 34 mm for the base and halved jointed as shown. Where 'A' is the distance from side to side and 'B' is the distance from front to rear. Pins are inserted through the bars along the length 'B', so that they protrude inward by approximately 15 mm

Now sandwich them all together and nail/screw

A thin sheet of plywood can now be slid under the mesh ... resting on the protruding nails, to monitor the mite fall

Now use a piece of 21 x 21 mm timber to make an entrance block