I am not quite sure whether or not I commented to this bottom board feature.
If you go to ABOUT.COM and check with the bb there you will find several
comments about the screened bottom boards.
The original idea came from central Europe and has been tested there for a
number of years. Dr Malcolm Sandford's APIS letter explained this matter in
detail.
I saw the advantage and convenience of this method immediately and proceded
to change all my bottom boards.Modifications were made to the original idea
and proves to be an even greater advantage.
The  modification idea could also be extended and it is my only regret that
the idea came to me only after all was done.
The result of these type of boards is without compare as far as I am
concerned.
A) Adjustable ventilation of the hive.
B) Ease of sticky stuff application to the bottom "Drawer" and repeated
   inspection.
C) Ease of cleaning without disturbing the colony.
D) Varroa extermination and count . (Varroas cannot return to the bees, the
   board itself has no influence or is a cause of varroa drop off).
E) Except for large items or debris, the 1/8 fabric is of the right size .
D) Bees appear not to be affected walking on the screen.

Construction:
Frame as per size of hivebox outside dimensions with 6 in extention in front
as landing.
Width of frame material : as you see fit, i.e. 1/1/4 to 1/1/2 inches.
Apply ( staple) 1/8 galvanised metal hardware cloth over the entire frame
dimension.
A 3/4 by 3/8 frame on top of the frame (and screen) with an opening for bee
entry of 5 inches or larger.
The depth of the frame material is also a question of your judgement but
should not be less the 1/1/2 inches below the sreen.
A removable sliding flat rectangle of 1/4 plywood ( Outdoor)within a groove
of the frame serves as ventilation adjustment and debris catcher.
An additional item could be another sliding frame for the screen to remove
larger item such as dead bees.
The 3/8 hight of the entry serves as mouse entry prevention.
Paint it all with a good enamel paint.( Paint stores carry returned paint
and sell for bargain prices.)
It helps if you have a friend with machinery or own it yourself.
The only other change I made is the feature of the broadside entry for the
bees.( 90 deg turned from the ordinary)
For those who live in areas of the infestation of the small hive beetle such
arrangement with an open screen , an elevated hive with an oilpan on the
ground would at least catch some of the beetles dropping through the hive
bottom sreen.
Now you can take, modify or leave it.
catfish